Forem: Aisalkyn Aidarova

THE MOST IMPORTANT CONCEPT: MEASURING RELIABILITY: SLO, SLA, SLI

Aisalkyn Aidarova — Thu, 07 May 2026 13:55:56 +0000

Site Reliability Engineering is not just monitoring or fixing servers.

It is:

Applying software engineering principles to operations to make systems reliable at scale.

That means:

You don’t manually fix things → you automate
You don’t guess → you measure
You don’t react → you design for failure

Core mindset

A normal engineer asks:

“Is the system working?”

An SRE asks:

“How well is it working, how often does it fail, and how much failure is acceptable?”

Before SRE existed, companies said:

System should be reliable

That means nothing.

SRE changed that to:

Reliability must be measurable

This is where SLI, SLO, SLA come in.

🧠 PART 3 — SLI (SERVICE LEVEL INDICATOR)

What it really is

An SLI is:

A real measurement of user experience

Not system metrics like CPU — but user-facing metrics.

Examples

Instead of:

CPU = 70%

We measure:

Request success rate
Request latency
Error rate

Real example

Imagine your API:

1000 requests
990 succeed
10 fail

Your SLI:

Success rate = 99%

Important rule

SLI must reflect USER experience

If user is unhappy → your SLI is wrong

🧠 PART 4 — SLO (SERVICE LEVEL OBJECTIVE)

What it really is

SLO is:

A target you set for your system performance

Example

You define:

99.9% of requests must succeed

That is your SLO.

Why SLO exists

Because perfection is impossible.

So instead of:

System must never fail ❌

We say:

System can fail within limits ✅

Another example

Latency SLO:

95% of requests < 200ms

Key idea

SLO defines acceptable reliability

🧠 PART 5 — SLA (SERVICE LEVEL AGREEMENT)

What it really is

SLA is:

Business contract based on SLO

Example

If uptime < 99.9% → customer gets refund

Important difference

Concept	Purpose
SLI	measurement
SLO	internal goal
SLA	external contract

🧠 PART 6 — ERROR BUDGET (THIS IS SENIOR LEVEL)

This is the most important concept in SRE.

What it is

If your SLO is:

99.9% uptime

Then:

0.1% failure is allowed

That is your error budget

In real time

~43 minutes downtime per month

Why it matters

It creates balance:

Developers → want speed
SRE → want stability

Error budget decides:

If budget remains → deploy
If exhausted → stop releases

Real rule

No error budget = no deployments

🧠 PART 7 — HOW WE MEASURE AVAILABILITY

Formula

Availability =

(Total time - downtime) / total time

Example

30 days = 720 hours
Downtime = 2 hours

(720 - 2) / 720 = 99.72%

SRE levels

Level	Meaning
99%	basic
99.9%	production
99.99%	critical
99.999%	extreme

🧠 PART 8 — LATENCY (WHY AVERAGE IS WRONG)

Average lies.

Example

99 requests = 100ms
1 request = 10 seconds

Average looks fine — but system is broken.

Solution

Use percentiles:

P50 → normal
P95 → slow users
P99 → worst users

Real SLO

95% of requests < 200ms

🧠 PART 9 — MONITORING (WHAT SRE ACTUALLY WATCHES)

Golden Signals (Google SRE)

Latency
Traffic
Errors
Saturation

What this means

You monitor:

How fast?
How many?
How broken?
How loaded?

Tools

Prometheus
Grafana
CloudWatch
ELK

🧠 PART 10 — ALERTING (VERY IMPORTANT)

Bad alert:

CPU > 80%

Good alert:

Error rate > 5% for 5 minutes

Rule

Alert only when users are impacted

🧠 PART 11 — INCIDENT MANAGEMENT

Incident = system failure affecting users

SRE process

Detect
Respond
Fix
Learn

Postmortem

Must be:

Blameless

You document

timeline
root cause
impact
fix
prevention

🧠 PART 12 — RELIABILITY ENGINEERING

You design systems that:

Expect failure

Example

Instead of 1 server:

ALB → multiple EC2 → DB replicas

Goal

No single point of failure

🧠 PART 13 — SCALING

Vertical

bigger machine

Horizontal

more machines

SRE prefers

Horizontal scaling

🧠 PART 14 — NETWORKING (WHAT YOU DID)

You must understand:

VPC
routing
NAT vs IGW
TGW
PrivateLink

🧠 PART 15 — AUTOMATION

Rule:

If you repeat it → automate it

Tools

Terraform
Bash
Python

🧠 PART 16 — CI/CD

You must know:

pipelines
deployments
rollback

Strategies

rolling
blue/green
canary

🧠 FINAL UNDERSTANDING

SRE is:

Measure → Define → Monitor → Improve → Automate

💬 PERFECT INTERVIEW ANSWER

SRE focuses on maintaining system reliability by defining measurable objectives like SLOs, monitoring system health, managing incidents, and automating infrastructure while balancing system stability with development velocity.

What is AWS PrivateLink?

Aisalkyn Aidarova — Thu, 07 May 2026 13:49:18 +0000

AWS PrivateLink lets you access AWS services or your own services hosted in another VPC privately — traffic never leaves the AWS network, never touches the internet, and the two VPCs don't need to be peered or connected via Transit Gateway.

The core idea: instead of exposing a service publicly or opening up full VPC-to-VPC networking, PrivateLink creates a one-way, private endpoint — the consumer VPC gets a private IP in its own subnet that tunnels traffic to the provider service. That's it. No route tables to manage, no CIDR conflicts to worry about.Diagram 1: How PrivateLink works — the core mechanism is an Interface Endpoint (an ENI in your subnet) that maps to the provider's service via AWS's internal network.The consumer VPC creates an Interface Endpoint — just a private IP (ENI) sitting in its own subnet. DNS resolves the service name to that private IP. Traffic flows through AWS's internal backbone to the provider's Network Load Balancer, then to the actual service. The two VPCs never need to peer, share route tables, or even know each other's CIDR ranges.

Diagram 2: The three ways PrivateLink is used in practice.---

The Three Uses of PrivateLink

1. Accessing AWS-managed services privately. Services like S3, SQS, ECR, Secrets Manager, KMS, and 100+ others support PrivateLink. Instead of your EC2 or Lambda hitting s3.amazonaws.com over the internet, an Interface Endpoint gives it a private IP inside your subnet. For S3 and DynamoDB specifically, there's a simpler free variant called a Gateway Endpoint — same idea, slightly different implementation.

2. Consuming a partner SaaS service. Vendors like Datadog, Splunk, Snowflake, and many others publish themselves as PrivateLink Endpoint Services. You create an Interface Endpoint in your VPC pointing to their service name, and your traffic to them never touches the internet. The vendor's VPC and your VPC never peer — they can't see your network at all, only receive the specific calls you make.

3. Publishing your own internal service. You place a Network Load Balancer in front of your service, register it as an Endpoint Service, then whitelist which AWS accounts can connect. Other teams or customers create Interface Endpoints in their own VPCs pointing at yours. This is how internal platform teams build shared services — auth, payments, data APIs — without opening full VPC-to-VPC access.

PrivateLink vs the Alternatives

	PrivateLink	VPC Peering	Transit Gateway
Traffic path	AWS backbone	AWS backbone	AWS backbone
CIDR conflicts	No problem	Breaks everything	No problem
Access scope	Single service only	Full VPC-to-VPC	All attached VPCs
Direction	One-way (consumer → provider)	Bidirectional	Bidirectional
Cross-account	Yes	Yes	Yes
Cross-region	Yes (via interface EP)	Yes	Yes (TGW peering)
Best for	Exposing a specific service privately	Small number of VPCs needing full access	Large-scale hub-and-spoke networking

The key insight: PrivateLink is surgical. VPC Peering and Transit Gateway open up networking — entire VPCs can talk to each other. PrivateLink exposes only one service through a single endpoint. If you just want your app to call an internal payments API without routing everything through a shared network, PrivateLink is exactly the right tool.

What is AWS Transit Gateway?

Transit Gateway (TGW) is a central network hub that connects multiple VPCs, on-premises networks, and AWS accounts together — like a cloud router that everything plugs into.

Think of it this way: without Transit Gateway, if you have 5 VPCs that all need to talk to each other, you'd need a mesh of VPC peering connections. With Transit Gateway, every VPC just connects to one hub.Diagram 1: The problem Transit Gateway solves — without it, VPCs connecting to each other require a full mesh of peering connections that grows unmanageable fast.The key pain point with VPC peering: it is non-transitive. If VPC A peers with VPC B, and VPC B peers with VPC C, VPC A still cannot talk to VPC C. You'd need a direct peering for every pair. With 10 VPCs that's 45 connections to manage. Transit Gateway fixes this entirely — everything connects through one hub.

Diagram 2: What Transit Gateway connects — it's not just VPC-to-VPC. It acts as the central router for your entire network.---

Why You Need Transit Gateway

1. Scale without the mesh chaos. VPC peering connections grow as N×(N-1)/2 — 4 VPCs need 6 connections, 10 VPCs need 45. TGW keeps it at N connections regardless.

2. Transitivity. VPC peering is not transitive — traffic can't hop through an intermediate VPC. TGW routes traffic across all attached networks as a proper router would.

3. Centralized on-premises connectivity. Without TGW, each VPC needs its own VPN tunnel to your data center. With TGW, one VPN attachment serves all VPCs attached to the gateway.

4. Traffic isolation via route tables. The TGW has its own route tables. You can define that your Dev VPC can only reach other Dev VPCs and a shared-services VPC, while a security VPC sees everything for traffic inspection. This is impossible with VPC peering alone.

5. Cross-account and cross-region. TGW works with AWS Resource Access Manager to share the gateway across multiple accounts. TGW peering connects gateways across regions.

TGW vs VPC Peering — When to Use Which

Scenario	Use
2–3 VPCs that need to talk	VPC Peering (simpler, cheaper)
4+ VPCs, especially growing	Transit Gateway
Cross-account, cross-region networking	Transit Gateway
On-premises + multiple VPCs	Transit Gateway
Need centralized firewall/inspection	Transit Gateway (route all traffic through security VPC)
Just two VPCs, same account	VPC Peering

The core mental model: VPC Peering is a direct cable between two VPCs. Transit Gateway is a router that every VPC plugs into. Once you have more than a handful of VPCs, the router approach wins every time.

Great question — this is where a lot of people get confused because all three keep traffic on the AWS backbone. The real difference is what problem each one solves.

Here's a decision framework:The one question that drives the choice: what scope of access do you need?---

The Core Mental Model

Think of it in terms of scope of access:

PrivateLink = expose one service to another VPC. The consumer gets a single private IP endpoint — nothing else. They cannot reach any other resource in your VPC. This is surgical, zero-trust access. Use it when two VPCs don't need to talk to each other broadly — they just need one service to be reachable.

VPC Peering = full network access between exactly two VPCs. Both VPCs can reach any resource in the other (subject to security groups and NACLs). Simple to set up, no cost for the connection itself, but does not scale — every new VPC pair needs its own peering, and it's non-transitive.

Transit Gateway = full network access between many VPCs, accounts, and on-premises networks, all routed through one hub. More complex and costs money per attachment + data processed, but scales linearly and supports centralized routing policies.

Real-World Scenarios

Scenario 1 — Startup with 2 VPCs (prod + dev)
Dev team occasionally needs to pull from a shared database in prod. → Use VPC Peering. Two VPCs, simple, done in minutes.

Scenario 2 — Company with 8 VPCs across 3 AWS accounts
Networking team needs all VPCs to reach a shared DNS resolver and a centralized logging service, plus a VPN back to the data center. → Use Transit Gateway. One VPN connection shared by all, one hub to manage routing.

Scenario 3 — Platform team building an internal payments API
Other teams' VPCs need to call the payments API — but the platform team doesn't want those VPCs to have any other access into their network. → Use PrivateLink. Expose only the API endpoint. Consumer VPCs get a single private IP, nothing more.

Scenario 4 — VPC Peering is impossible (overlapping CIDRs)
Two VPCs both use 10.0.0.0/16 — peering is blocked. But one VPC needs to call a service in the other. → Use PrivateLink. CIDR conflicts don't matter since there's no route table overlap.

Scenario 5 — Lambda in a private VPC needs to write to S3
Lambda is in a VPC with no internet access. You need to reach S3 without adding a NAT Gateway. → Use PrivateLink (Gateway Endpoint for S3 — free). Traffic stays entirely within AWS.

Quick Reference

	PrivateLink	VPC Peering	Transit Gateway
Access scope	Single service	Whole VPC	Whole network
CIDR conflicts	No issue	Breaks it	No issue
Scale	Unlimited consumers	Up to ~125 peers	Thousands of VPCs
On-premises	No	No	Yes (VPN/DX)
Cost	Per endpoint + data	Data transfer only	Per attachment + data
Direction	One-way	Bidirectional	Bidirectional
Complexity	Low	Very low	Medium-high

The pattern most large companies end up with: Transit Gateway as the backbone for VPC-to-VPC and on-premises connectivity, with PrivateLink layered on top for exposing specific internal services securely to teams or customers who shouldn't have broad network access.

Cloud Service Models — Full SRE Lecture: IaaS, PaaS, SaaS

Aisalkyn Aidarova — Thu, 07 May 2026 13:47:09 +0000

🌐 The Big Picture First

Think of cloud service models as a spectrum of responsibility. The more you move right, the less you manage — but also the less control you have.

YOUR RESPONSIBILITY
◄────────────────────────────────────────────►
Maximum                                Minimum

On-Premises → IaaS → PaaS → SaaS → Serverless

A useful analogy is pizza:

On-Premises  = Make pizza at home (you own everything)
IaaS         = Order dough & ingredients (you cook it)
PaaS         = Order pizza kit (just assemble & bake)
SaaS         = Order delivery (just eat it)

🏗️ Layer 1 — IaaS (Infrastructure as a Service)

What it is

You rent raw infrastructure — servers, storage, networking — from a cloud provider. The provider manages the physical hardware. You manage everything above the hypervisor.

Responsibility Split

Provider manages:        You manage:
─────────────────        ───────────────────────
Physical servers    →    Operating System
Data centers        →    Runtime & middleware
Networking HW       →    Applications
Hypervisor          →    Data
Storage HW          →    Security patches
                    →    Scaling
                    →    Backups
                    →    Monitoring

Real Examples

Provider	IaaS Products
AWS	EC2, EBS, VPC, S3
GCP	Compute Engine, Cloud Storage
Azure	Virtual Machines, Azure Blob

IaaS Use Cases

Lift & shift migrations from on-prem
Custom OS configurations needed
High performance computing (HPC)
Full control over networking required
Legacy applications that can't be containerized

IaaS Code Example — Terraform EC2

# You provision and OWN this — classic IaaS
resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.medium"

  # YOU are responsible for everything inside this machine
  user_data = <<-EOF
    #!/bin/bash
    apt-get update
    apt-get install -y nginx
    systemctl start nginx
  EOF

  tags = {
    Name = "company-web-server"
  }
}

🚀 Layer 2 — PaaS (Platform as a Service)

What it is

The provider manages OS, runtime, middleware, and scaling. You focus purely on writing and deploying your application code.

Responsibility Split

Provider manages:        You manage:
─────────────────        ───────────────────────
Physical servers    →    Application code
Data centers        →    Data
Hypervisor          →    User access
Operating System    →    Configurations
Runtime             →    (sometimes) scaling rules
Middleware          →
Patching            →
Scaling infra       →

Real Examples

Provider	PaaS Products
AWS	Elastic Beanstalk, RDS, Lambda
GCP	App Engine, Cloud Run, Cloud SQL
Azure	Azure App Service, Azure SQL
Others	Heroku, Render, Railway

PaaS Use Cases

Startups moving fast without dedicated DevOps
Managed databases (RDS handles patching, backups)
Web apps where you don't care about OS
Rapid prototyping

PaaS Code Example — AWS Elastic Beanstalk

# You just push code — platform handles the rest
# .ebextensions/app.config

option_settings:
  aws:autoscaling:asg:
    MinSize: 2
    MaxSize: 10
  aws:elasticbeanstalk:environment:
    EnvironmentType: LoadBalanced
  aws:ec2:instances:
    InstanceTypes: t3.medium

# No OS management, no nginx config, no patching
# Platform handles it ALL

💻 Layer 3 — SaaS (Software as a Service)

What it is

A fully managed application delivered over the internet. You don't manage infrastructure, OS, runtime, or the app itself. You just use it.

Responsibility Split

Provider manages:        You manage:
─────────────────        ───────────────────────
Everything               Your data
Infrastructure      →    User access/permissions
OS & runtime        →    Configurations within app
Application         →    Integrations
Updates & patches   →
Scaling             →
Security            →

Real Examples

Category	SaaS Products
Monitoring	Datadog, New Relic, PagerDuty
Communication	Slack, Gmail, Zoom
CI/CD	GitHub Actions, CircleCI
Security	Okta, CrowdStrike
Storage	Dropbox, Google Drive

SaaS from SRE perspective

You don't manage the app BUT you must manage:
✅ API integrations with your systems
✅ SSO/SAML configuration
✅ Data retention policies
✅ Vendor SLA monitoring
✅ Cost & license management
✅ Data backup (vendor may not guarantee YOUR data)

🔄 Shared Responsibility Model — Deep Dive

This is critical for SRE engineers to understand deeply.

                    ON-PREM   IaaS    PaaS    SaaS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Applications          YOU      YOU     YOU    VENDOR
Data                  YOU      YOU     YOU     YOU ⚠️
Runtime               YOU      YOU    VENDOR  VENDOR
Middleware            YOU      YOU    VENDOR  VENDOR
OS                    YOU      YOU    VENDOR  VENDOR
Virtualization        YOU     VENDOR  VENDOR  VENDOR
Servers               YOU     VENDOR  VENDOR  VENDOR
Storage               YOU     VENDOR  VENDOR  VENDOR
Networking            YOU     VENDOR  VENDOR  VENDOR
Data Center           YOU     VENDOR  VENDOR  VENDOR
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

⚠️ Data is always YOUR responsibility regardless of model. Even in SaaS, if vendor loses your data — that's your problem operationally.

🧠 What an SRE Must Know About Each Model

IaaS — SRE Responsibilities

1. OS Hardening & Patching

# You own this with IaaS
# Automated patching with SSM
aws ssm send-command \
  --document-name "AWS-RunPatchBaseline" \
  --targets "Key=tag:Environment,Values=production" \
  --parameters '{"Operation":["Install"]}'

2. Auto Scaling & Self Healing

resource "aws_autoscaling_group" "web" {
  min_size         = 2
  max_size         = 20
  desired_capacity = 4

  health_check_type         = "ELB"
  health_check_grace_period = 300

  # Self-healing — replace unhealthy instances automatically
  instance_refresh {
    strategy = "Rolling"
    preferences {
      min_healthy_percentage = 50
    }
  }
}

3. Monitoring You Must Set Up Yourself

# Prometheus scrape config for IaaS EC2
scrape_configs:
  - job_name: 'ec2-instances'
    ec2_sd_configs:
      - region: us-east-1
        port: 9100  # node_exporter port
    relabel_configs:
      - source_labels: [__meta_ec2_tag_Environment]
        target_label: environment

4. Backup Strategy

# EBS snapshots — YOUR responsibility in IaaS
aws ec2 create-snapshot \
  --volume-id vol-xxxxxxxx \
  --description "Daily backup $(date +%Y-%m-%d)"

PaaS — SRE Responsibilities

1. Monitor What the Platform Exposes

# RDS is PaaS — you monitor metrics, not the OS
import boto3

cloudwatch = boto3.client('cloudwatch')

# Key RDS metrics to alert on
rds_metrics = [
    'CPUUtilization',        # > 80% = alert
    'FreeStorageSpace',      # < 20% = alert
    'DatabaseConnections',   # near max = alert
    'ReadLatency',           # > 20ms = investigate
    'WriteLatency',          # > 20ms = investigate
    'ReplicaLag',            # > 30s = alert
]

2. Understand Platform Limits

AWS RDS Limits you MUST know as SRE:
- Max connections per instance type
- Storage autoscaling thresholds
- Failover time (~60-120 seconds)
- Backup retention (1-35 days)
- Maintenance windows impact

If you don't know these → you'll miss incidents

3. Runbook for PaaS Failures

## RDS Failover Runbook

1. Alert fires: RDS_ReplicaLag > 30s
2. Check: AWS Console → RDS → Events
3. If primary unhealthy → failover triggers automatically
4. Expected downtime: 60-120 seconds
5. Verify: application reconnects (check connection pooling)
6. Notify: stakeholders if > 2 min downtime
7. Postmortem: if failover was unexpected

SaaS — SRE Responsibilities

1. Vendor SLA Tracking

# Track your SaaS vendors' uptime against their SLA
vendors = {
    "datadog": {
        "sla_target": 99.9,
        "status_page": "https://status.datadoghq.com",
        "impact": "CRITICAL"  # no monitoring if down
    },
    "pagerduty": {
        "sla_target": 99.9,
        "status_page": "https://status.pagerduty.com",
        "impact": "CRITICAL"  # no alerting if down
    },
    "github": {
        "sla_target": 99.9,
        "status_page": "https://githubstatus.com",
        "impact": "HIGH"  # no deploys if down
    }
}

2. SaaS Dependency Risk

As SRE you must ask:
❓ What happens if Datadog goes down?
   → Do we have fallback monitoring?

❓ What happens if PagerDuty goes down?
   → Do we have SMS/phone tree backup?

❓ What happens if GitHub goes down?
   → Can we still deploy hotfixes?

❓ What happens if Okta goes down?
   → Can engineers still access production?

3. Data Backup for SaaS

# Even SaaS data needs backup — vendor not responsible
# Example: backup GitHub repos

#!/bin/bash
ORGS=("company-org")
for org in "${ORGS[@]}"; do
  repos=$(gh repo list $org --json name -q '.[].name')
  for repo in $repos; do
    git clone --mirror \
      https://github.com/$org/$repo.git \
      /backups/github/$org/$repo.git
  done
done

📊 SLO/SLI Design Per Model

This is where SRE expertise really shows:

IaaS — You define AND measure everything:
  SLI: Custom metrics from your app + infra
  SLO: 99.9% availability (you control this)
  Error Budget: You own it fully

PaaS — Platform gives you some metrics:
  SLI: Mix of platform metrics + app metrics
  SLO: Limited by platform's own SLA
  Error Budget: Platform failures count against YOU

SaaS — You mostly observe:
  SLI: API response times, login success rate
  SLO: Constrained by vendor SLA
  Error Budget: Vendor downtime burns YOUR budget

🔥 Real Incident Scenarios by Model

IaaS Incident

Alert: High CPU on EC2 fleet (95%)
SRE Actions:
1. SSH into instance → top → find runaway process
2. Check ASG → is it scaling?
3. Check ALB → redistribute traffic
4. Patch if OS-level issue
5. You have FULL access to diagnose

Resolution time: Fast if skilled, slow if not

PaaS Incident

Alert: RDS connections maxed out
SRE Actions:
1. Check CloudWatch → DatabaseConnections metric
2. Check application → connection pool config
3. Scale instance type (few minutes)
4. Add read replica to distribute load
5. You CANNOT ssh into RDS — limited visibility

Resolution time: Dependent on platform tooling

SaaS Incident

Alert: Datadog not receiving metrics
SRE Actions:
1. Check status.datadoghq.com
2. Check your agent → is it running?
3. If vendor issue → wait + use backup monitoring
4. You have ZERO control over their infrastructure

Resolution time: Entirely up to vendor

💡 Key SRE Takeaways

Topic	IaaS	PaaS	SaaS
Toil level	High	Medium	Low
Control	Full	Partial	None
Blast radius	You caused it	Shared	Vendor caused it
MTTR	You control	Partly you	Vendor controls
Cost model	Pay per resource	Pay per usage	Pay per seat
Scaling	Manual/ASG	Auto	Automatic
Patching	You	Vendor	Vendor
Debugging	Full access	Limited	API/logs only

🎓 Senior SRE Mental Model

At 6 years experience, you should think about this like:

IaaS = Maximum flexibility, maximum toil
       → Use when you NEED control
       → Automate everything or drown

PaaS = Sweet spot for most workloads
       → Understand platform limits deeply
       → Know exactly what you can't control

SaaS = Treat vendors like internal services
       → Track their SLAs
       → Build fallbacks for critical ones
       → Own YOUR data always

Modern SRE reality:
Most companies use ALL THREE simultaneously
Your job = understand the boundary of responsibility
           at each layer and build reliability
           within those constraints

What is a VPC in AWS? VPC peering, transit

Aisalkyn Aidarova — Wed, 06 May 2026 20:48:13 +0000

VPC (Virtual Private Cloud) is your own logically isolated network within AWS — think of it as your private data center inside AWS's infrastructure, where you control the IP ranges, subnets, routing, and security.

Why Do We Need a VPC?

Without a VPC, all your AWS resources would be on a shared public network — anyone could potentially reach them. VPC solves this by:

Isolation — your resources are invisible to other AWS accounts
Security — you control what traffic comes in and goes out
Custom networking — define your own IP ranges, subnets, and routes
Compliance — meet regulatory requirements by keeping data in private networks

Key Components of a VPC---

Key VPC Building Blocks

Subnets divide your VPC into sections. A public subnet has a route to the Internet Gateway, so resources there can receive inbound traffic. A private subnet has no direct internet route — resources there are unreachable from outside unless you explicitly allow it.

Internet Gateway (IGW) is the front door. It attaches to your VPC and allows two-way communication with the internet — but only for resources in public subnets that also have a public IP.

NAT Gateway lets private-subnet resources (like your database) make outbound calls (e.g. downloading patches) without exposing them to inbound internet traffic. Traffic flows: Private EC2 → NAT GW → IGW → Internet, but never the reverse.

Route Tables are the GPS of your VPC. Each subnet is associated with a route table that tells AWS where to send traffic — public subnets route 0.0.0.0/0 to the IGW, private subnets route it to the NAT GW.

Security Groups act as virtual firewalls at the instance level — you define which ports and IPs are allowed in/out for each resource.

VPC Endpoints let services like Lambda or EC2 talk to S3, DynamoDB, or Secrets Manager without traffic leaving AWS's backbone — no IGW, no NAT, faster and cheaper.

How Services Connect to a VPC

Service	How it connects
EC2	Launched directly inside a subnet — has a private IP, optionally a public one
RDS	Placed in a DB subnet group (typically 2+ private subnets across AZs)
Lambda	By default runs outside any VPC; you can attach it to a VPC for private access
ECS / EKS	Tasks/pods run inside subnets like EC2
S3 / DynamoDB	Public services; access via VPC Endpoint keeps traffic private
ALB	Lives in public subnets, forwards to private-subnet targets

Public vs Private — When to Use Which

Use a public subnet for: load balancers, bastion hosts, NAT Gateways — anything that genuinely needs to receive internet traffic.

Use a private subnet for: databases, application servers, Lambda functions, internal microservices — anything that should never be directly reachable from the internet.

The general rule: put as little as possible in the public subnet. The smaller your public surface, the harder it is to attack.

What is AWS Transit Gateway?

Transit Gateway (TGW) is a central network hub that connects multiple VPCs, on-premises networks, and AWS accounts together — like a cloud router that everything plugs into.

Diagram 2: What Transit Gateway connects — it's not just VPC-to-VPC. It acts as the central router for your entire network.---

Why You Need Transit Gateway

1. Scale without the mesh chaos. VPC peering connections grow as N×(N-1)/2 — 4 VPCs need 6 connections, 10 VPCs need 45. TGW keeps it at N connections regardless.

2. Transitivity. VPC peering is not transitive — traffic can't hop through an intermediate VPC. TGW routes traffic across all attached networks as a proper router would.

3. Centralized on-premises connectivity. Without TGW, each VPC needs its own VPN tunnel to your data center. With TGW, one VPN attachment serves all VPCs attached to the gateway.

5. Cross-account and cross-region. TGW works with AWS Resource Access Manager to share the gateway across multiple accounts. TGW peering connects gateways across regions.

TGW vs VPC Peering — When to Use Which

Scenario	Use
2 VPCs that need to talk	VPC Peering (simpler, cheaper)
4+ VPCs, especially growing	Transit Gateway
Cross-account, cross-region networking	Transit Gateway
On-premises + multiple VPCs	Transit Gateway
Need centralized firewall/inspection	Transit Gateway (route all traffic through security VPC)
Just two VPCs, same account	VPC Peering

VPC Peering is a networking connection between two Virtual Private Clouds (VPCs) that allows them to communicate with each other using private IP addresses — as if they were on the same network.

Key ideas:

VPC = an isolated private network within a cloud provider (like AWS, Google Cloud, or Azure)
Peering = linking two of those networks together directly, without traffic going over the public internet

How it works:

Traffic between peered VPCs travels through the cloud provider's internal backbone network, making it fast, private, and secure.

Common use cases:

Connecting a development VPC to a production VPC
Sharing services (like a database) across teams or accounts
Connecting VPCs across different regions or different accounts within the same cloud provider

Important limitations:

Non-transitive — if VPC A peers with VPC B, and VPC B peers with VPC C, VPC A cannot talk to VPC C through B. Each connection must be explicitly set up.
No overlapping CIDR blocks — the IP address ranges of the two VPCs cannot overlap
Not a VPN — it's a private cloud-internal connection, not an encrypted tunnel over the internet

Quick analogy:

Think of two office buildings (VPCs) in the same city. VPC peering is like building a private hallway directly between them, so employees can walk between buildings without going outside (the public internet).

Want a diagram or a deeper dive into any specific cloud provider's implementation?
Great question — this is where a lot of people get confused because all three keep traffic on the AWS backbone. The real difference is what problem each one solves.

Here's a decision framework:The one question that drives the choice: what scope of access do you need?---

The Core Mental Model

Think of it in terms of scope of access:

Real-World Scenarios

Scenario 1 — Startup with 2 VPCs (prod + dev)
Dev team occasionally needs to pull from a shared database in prod. → Use VPC Peering. Two VPCs, simple, done in minutes.

Quick Reference

	PrivateLink	VPC Peering	Transit Gateway
Access scope	Single service	Whole VPC	Whole network
CIDR conflicts	No issue	Breaks it	No issue
Scale	Unlimited consumers	Up to ~125 peers	Thousands of VPCs
On-premises	No	No	Yes (VPN/DX)
Cost	Per endpoint + data	Data transfer only	Per attachment + data
Direction	One-way	Bidirectional	Bidirectional
Complexity	Low	Very low	Medium-high

What is AWS PrivateLink?

Diagram 2: The three ways PrivateLink is used in practice.---

The Three Uses of PrivateLink

PrivateLink vs the Alternatives

	PrivateLink	VPC Peering	Transit Gateway
Traffic path	AWS backbone	AWS backbone	AWS backbone
CIDR conflicts	No problem	Breaks everything	No problem
Access scope	Single service only	Full VPC-to-VPC	All attached VPCs
Direction	One-way (consumer → provider)	Bidirectional	Bidirectional
Cross-account	Yes	Yes	Yes
Cross-region	Yes (via interface EP)	Yes	Yes (TGW peering)
Best for	Exposing a specific service privately	Small number of VPCs needing full access	Large-scale hub-and-spoke networking

project #1: Company: *FinTrust Bank (digital banking platform) your role: 👉 Site Reliability Engineer (SRE)

Aisalkyn Aidarova — Thu, 30 Apr 2026 00:25:26 +0000

What bank do:

Online banking (accounts, transfers, payments)
Mobile + web applications
Real-time transactions
Strict security & compliance (PCI-DSS, encryption)

👩‍💻 YOUR ROLE

Title:

👉 Site Reliability Engineer (SRE)

Your responsibility

Ensure 99.99% uptime
Protect sensitive financial data
Prevent unauthorized access
Ensure low latency transactions
Handle incidents quickly
Maintain secure architecture

🏗️ PROJECT NAME

👉 Secure Multi-Tier Banking Infrastructure on AWS with High Availability and Zero-Trust Networking

🧠 CORE IDEA

Banking system MUST:

✔ Never expose database
✔ Encrypt all traffic
✔ Restrict access strictly
✔ Handle failures instantly
✔ Be fully observable
✔ Support multi-region design

🏗️ ARCHITECTURE

User (Mobile / Web)
   ↓
DNS (:contentReference[oaicite:0]{index=0})
   ↓
:contentReference[oaicite:1]{index=1} + Shield
   ↓
CloudFront (CDN + TLS)
   ↓
Application Load Balancer (DMZ / Public)
   ↓
App Layer (Private Subnets)
   ↓
Transaction Services (Private)
   ↓
Database (Private DB Subnet, encrypted)

🔐 SECURITY (MOST IMPORTANT FOR BANK)

What you implemented

1. Network isolation

VPC with private architecture
No public IPs for app or DB
Only ALB exposed

2. Firewall design

ALB SG → allow 443 from internet
App SG → allow only from ALB
DB SG → allow only from app

👉 Zero trust model

3. Encryption

HTTPS everywhere (TLS)
DB encryption (at rest)
Secrets stored securely

4. WAF protection

blocked SQL injection
blocked bots
rate limiting

🌐 NETWORKING (WHAT YOU BUILT)

VPC design

10.0.0.0/16

Subnets:

Public (DMZ):
- ALB
- NAT

Private App:
- Banking APIs

Private DB:
- RDS (transactions)

Routing

Public route table:

0.0.0.0/0 → IGW

Private route table:

0.0.0.0/0 → NAT

DB route table:

NO internet access

Private access

Used:

VPC Endpoint for S3
VPC Endpoint for Secrets Manager

👉 No internet dependency

⚖️ HIGH AVAILABILITY (BANK REQUIREMENT)

Multi-AZ deployment
ALB distributes traffic
Auto Scaling enabled

Failure handling

If one AZ fails:

Traffic shifts automatically

📡 MULTI-VPC / ENTERPRISE DESIGN

You designed:

Core banking VPC
Shared services VPC

Connected using:

VPC Peering
AWS Transit Gateway

🔒 PRIVATELINK (VERY STRONG POINT)

Used:

AWS PrivateLink

Use case:

internal fraud detection API exposed privately

👉 No full VPC exposure

🏢 HYBRID (REAL BANKING)

Bank has on-prem systems:

legacy transaction systems

Connected using:

VPN
Direct Connect (concept)

📊 OBSERVABILITY (SRE CORE)

You implemented:

CloudWatch metrics
ALB access logs
VPC Flow Logs

What you monitor

latency
error rate
traffic spikes
blocked requests
DB connections

🚨 INCIDENTS YOU HANDLED

Example 1 — Payment API down

ALB 503
found unhealthy targets
restarted service
fixed health check

Example 2 — Transaction delay

high latency detected
traced to DB slow query
optimized query

Example 3 — Security alert

WAF blocked traffic spike
identified bot attack
tuned rules

Example 4 — Private EC2 lost internet

NAT route missing
fixed route table

Example 5 — DNS misrouting

wrong ALB target
updated Route 53

🧑‍🤝‍🧑 TEAM STRUCTURE

2 SREs
5 backend engineers
2 frontend engineers
1 security engineer
1 DevOps/platform engineer

🤝 YOUR COLLABORATION

You worked with:

backend → debugging API failures
security → WAF rules, compliance
DevOps → deployments
product → outage impact

📅 YOUR DAILY WORK

Morning:

check dashboards
review alerts

During day:

fix incidents
optimize performance
deploy updates

On-call:

respond to outages
troubleshoot quickly

🏆 YOUR ACHIEVEMENTS

You can say:

achieved 99.99% uptime
reduced downtime by resolving recurring issues
secured architecture (no public DB)
improved performance
reduced costs using VPC endpoints

💬 STRONG INTERVIEW ANSWER

Say this:

“I worked as an SRE on a banking platform where I designed and maintained a secure multi-tier AWS architecture. I implemented private networking using VPC, subnets, and NAT Gateway, and ensured that only the load balancer was exposed publicly. I secured communication using security groups and WAF, and placed the database in isolated private subnets with no internet access. I integrated DNS using Route 53 and implemented private access to AWS services using VPC endpoints. I also designed multi-VPC connectivity using Transit Gateway and PrivateLink for secure service exposure. As part of my SRE responsibilities, I monitored system health using CloudWatch and logs, handled incidents such as load balancer failures and database connectivity issues, and ensured high availability and performance for critical banking transactions.”

🔥 WHY THIS PROJECT IS POWERFUL

Because it shows:

✔ Security (bank-level)
✔ Networking (deep)
✔ Reliability (SRE core)
✔ Real-world scenarios
✔ Troubleshooting

VPC, subnets, IGW, NAT, routing, firewall, DMZ, private DB, and troubleshooting part #3

Aisalkyn Aidarova — Thu, 30 Apr 2026 00:18:11 +0000

Real Outage Simulation: SRE Networking Debugging

Architecture:

User
 ↓
Route 53 DNS
 ↓
ALB public subnet / DMZ
 ↓
Web EC2 private subnet
 ↓
DB private subnet

Your SRE troubleshooting order:

1. DNS
2. WAF / ALB
3. Target Group health
4. Security Groups
5. Route Tables
6. NAT / IGW
7. EC2 / Nginx
8. DB
9. Logs / Flow Logs

OUTAGE 1 — Website Completely Down

Symptom

User says:

app.company.com is not opening.

Browser shows:

This site can’t be reached

Step 1 — Check DNS

From your laptop:

nslookup app.company.com

Expected good output:

Name: app.company.com
Address: ALB-DNS or ALB IPs

Bad output:

server can't find app.company.com

Root cause

Route 53 record deleted or wrong.

Fix

Go to:

Route 53 → Hosted Zone → Create Record

Create:

Record type: A
Alias: Yes
Target: Application Load Balancer

SRE explanation

DNS was not resolving to the ALB, so traffic never reached AWS infrastructure.

OUTAGE 2 — ALB Returns 503

Symptom

Browser opens, but shows:

503 Service Temporarily Unavailable

Meaning

ALB is reachable, but it has no healthy backend targets.

Step 1 — Check Target Group

Go to:

EC2 → Target Groups → sre-app-tg → Targets

Bad output:

Unhealthy

Step 2 — Check health reason

Possible reasons:

Health checks failed
Request timed out
Target.ResponseCodeMismatch

Step 3 — Check app server

Connect to private EC2 using SSM or bastion.

Run:

sudo systemctl status nginx

Bad output:

inactive (dead)

Fix

sudo systemctl start nginx
sudo systemctl enable nginx

Check:

curl localhost

Expected:

Hello from Web Server 1

SRE explanation

ALB returned 503 because the target group had no healthy instances. The web service was stopped, so the health check failed.

OUTAGE 3 — ALB Target Unhealthy Because Security Group Is Wrong

Symptom

ALB returns:

Target group shows:

Unhealthy
Health check timeout

Check Security Group

Go to:

EC2 → Security Groups → web-sg → Inbound rules

Correct rule should be:

HTTP 80 from alb-sg

Bad rule example:

HTTP 80 from your IP

Root cause

ALB cannot reach web server because web SG does not allow traffic from ALB SG.

Fix

Edit web-sg inbound:

Type: HTTP
Port: 80
Source: alb-sg

Wait 1–2 minutes.

Expected:

Target health: Healthy

SRE explanation

The application itself was fine, but the firewall blocked ALB-to-web traffic.

OUTAGE 4 — Private EC2 Cannot Install Packages

Symptom

On private EC2:

sudo apt update

Bad output:

Temporary failure resolving
or
Connection timed out

Step 1 — Check route table

Go to:

VPC → Route Tables → private-rt → Routes

Correct:

0.0.0.0/0 → NAT Gateway

Bad:

No default route

Step 2 — Check NAT

Go to:

VPC → NAT Gateways

Expected:

State: Available
Subnet: Public subnet
Elastic IP: attached

Step 3 — Check public route table

Public subnet must have:

0.0.0.0/0 → Internet Gateway

Fix

Add route:

Private Route Table
0.0.0.0/0 → NAT Gateway

SRE explanation

Private EC2 had no outbound internet because the private subnet was missing the NAT route.

OUTAGE 5 — Public EC2 / ALB Not Reachable

Symptom

Browser cannot reach ALB DNS.

Check ALB Security Group

Go to:

EC2 → Security Groups → alb-sg

Correct inbound:

HTTP 80 from 0.0.0.0/0

Bad:

No inbound rule

Fix

Add:

HTTP 80 → 0.0.0.0/0

SRE explanation

The ALB was healthy, but its security group blocked public HTTP traffic.

OUTAGE 6 — Web Server Cannot Connect to DB

Symptom

Application shows:

Database connection failed

Step 1 — From web EC2 test DB port

nc -vz <db-private-ip> 3306

Bad output:

connection timed out

Good output:

succeeded

Step 2 — Check DB SG

Correct inbound rule:

MySQL 3306 from web-sg

Bad rule:

MySQL 3306 from your IP
or
No MySQL rule

Fix

Edit db-sg:

Inbound:
MySQL/Aurora
Port: 3306
Source: web-sg

SRE explanation

The database was private and secure, but the app tier was not allowed by the DB security group.

OUTAGE 7 — One Web Server Down, Site Still Works

Symptom

Stop one EC2:

sre-web-1 stopped

User still sees website.

Why?

ALB sends traffic only to healthy targets.

Check:

Target group:
web-1 → unused/unhealthy
web-2 → healthy

SRE explanation

This proves high availability. One instance failed, but ALB removed it from rotation and continued sending traffic to the healthy instance.

OUTAGE 8 — Both Web Servers in Same AZ

Symptom

Application works normally, but during AZ failure everything goes down.

Root cause

Both web servers are in one Availability Zone.

Bad design:

web-1 → us-east-1a
web-2 → us-east-1a

Good design:

web-1 → us-east-1a
web-2 → us-east-1b

Fix

Launch web servers across different private subnets in different AZs.

SRE explanation

High availability requires spreading resources across multiple Availability Zones.

OUTAGE 9 — Wrong Health Check Path

Symptom

Website works manually:

curl http://private-ip

Output:

Hello from Web Server

But ALB target is unhealthy.

Check health check path

Go to:

Target Group → Health checks

Bad path:

/health

But app only serves:

Fix

Change health check path to:

Or create /health endpoint.

SRE explanation

The application was running, but ALB health check was using a path that did not exist.

OUTAGE 10 — NACL Blocks Return Traffic

Symptom

Security groups look correct. Route tables look correct. Still traffic times out.

Check NACL

Go to:

VPC → Network ACLs

Remember:

NACL is stateless
Inbound and outbound both must be allowed

For HTTP, allow:

Inbound:

80 from source
1024-65535 ephemeral ports

Outbound:

80
1024-65535 ephemeral ports

Root cause

NACL allowed inbound request but blocked return traffic.

Fix

Allow ephemeral ports.

SRE explanation

Security groups are stateful, but NACLs are stateless. Return traffic must be explicitly allowed.

OUTAGE 11 — VPC Peering Not Working

Symptom

EC2 in VPC A cannot reach EC2 in VPC B.

Check 1 — Peering status

VPC → Peering Connections

Expected:

Active

Bad:

Pending acceptance

Check 2 — Route tables

VPC A route table:

10.1.0.0/16 → peering connection

VPC B route table:

10.0.0.0/16 → peering connection

Check 3 — CIDR overlap

Bad:

VPC A: 10.0.0.0/16
VPC B: 10.0.0.0/16

Peering will not work.

SRE explanation

VPC peering requires non-overlapping CIDR ranges, active peering, routes on both sides, and firewall rules allowing traffic.

OUTAGE 12 — PrivateLink Works for One Service Only

Symptom

Consumer VPC can access one API through endpoint, but cannot reach other private EC2s in provider VPC.

Explanation

This is expected.

PrivateLink is not full network connectivity.

PrivateLink → service-level access
VPC Peering → network-level access
Transit Gateway → large network hub

SRE explanation

PrivateLink exposes only a specific service through an endpoint. It does not allow full VPC-to-VPC communication.

OUTAGE 13 — VPN Tunnel Down

Symptom

On-prem users cannot reach AWS private app.

Check

In AWS:

VPC → Site-to-Site VPN Connections

Bad output:

Tunnel 1: DOWN
Tunnel 2: DOWN

Check:

Customer gateway public IP correct?
On-prem firewall allows IPsec?
BGP routes advertised?
AWS route table has route to on-prem CIDR?

SRE explanation

VPN issues usually come from tunnel status, BGP route advertisement, firewall rules, or missing route table entries.

OUTAGE 14 — DNS Points to Old ALB

Symptom

New deployment completed, but users still hit old app.

Check

dig app.company.com

Compare with current ALB DNS.

Root cause

Route 53 record points to old ALB or DNS cache TTL has not expired.

Fix

Update Route 53 alias record.

Lower TTL before planned migration.

SRE explanation

The application was not broken. DNS was pointing users to the wrong load balancer.

OUTAGE 15 — WAF Blocks Real Users

Symptom

Some users get:

403 Forbidden

Check

Go to:

AWS WAF → Web ACL → Logs / Sampled requests

Look for:

Blocked rule
Source IP
URI path
User agent

Fix

Options:

Adjust managed rule
Add allowlist
Change rule priority
Tune rate limit

SRE explanation

WAF protects the app, but rules can create false positives. SRE must verify blocked requests before disabling protection.

Final SRE Outage Debugging Script

In interview, say:

When an outage happens, I do not guess. I follow the request path. First I check DNS resolution, then ALB status, listener, security group, target group health, application service, route tables, NAT or IGW, then database connectivity. I also use ALB logs, VPC Flow Logs, CloudWatch metrics, and application logs to prove where the traffic is failing.

Best Practice Summary

DNS issue       → dig / nslookup
ALB issue       → listener, SG, target group
503             → no healthy targets
504             → backend timeout
Private no net  → NAT route
DB issue        → DB SG from web SG
NACL issue      → remember stateless
Peering issue   → routes both sides
VPN issue       → tunnel + BGP + routes
WAF issue       → check blocked rules

Very strong interview answer

I troubleshoot production outages by following the traffic path from user to backend: DNS, WAF, load balancer, target group, security groups, route tables, NACLs, EC2 service, and database. I verify each layer with tools like dig, curl, nc, ALB health checks, CloudWatch metrics, ALB logs, and VPC Flow Logs. My goal is to quickly identify whether the problem is DNS, routing, firewall, load balancer, application, or database, then restore service and document the root cause.

VPC, subnets, IGW, NAT, routing, firewall, DMZ, private DB, and troubleshooting part #2

Aisalkyn Aidarova — Thu, 30 Apr 2026 00:16:02 +0000

User (Internet)
   ↓
DNS (:contentReference[oaicite:0]{index=0})
   ↓
WAF (optional)
   ↓
Load Balancer (Public / DMZ)
   ↓
Private Web Tier (EC2 / App)
   ↓
Private DB Tier
   ↓
Private AWS Services (via VPC Endpoint)

Cross-VPC / Hybrid:
   ↔ VPC Peering / :contentReference[oaicite:1]{index=1}
   ↔ :contentReference[oaicite:2]{index=2}
   ↔ VPN / Direct Connect

🚀 STEP 11 — ADD DNS (Route 53)

Why SRE adds this

Users should never access ALB DNS directly.
They use domain like:

app.company.com

Go to:

Route 53 → Hosted Zones → Create Hosted Zone

If you already have domain → use it

Create record

Record name: app
Type: A
Alias: YES
Target: ALB

Expected result

nslookup app.yourdomain.com

Output:

Name: app.yourdomain.com
Address: ALB IP

Why this matters

Now flow is:

User → DNS → ALB → Web

SRE troubleshooting

If site down:

dig app.yourdomain.com

Check:

does it resolve?
correct ALB?
TTL delay?

🚀 STEP 12 — ADD WAF (SECURITY LAYER)

Why

Security Groups = network firewall
WAF = application firewall (Layer 7)

Go to:

WAF → Create Web ACL

Attach to:

ALB

Add rules

AWS Managed Rules
Rate limiting (1000 req/min)

Result

Now:

Bad traffic blocked BEFORE app

SRE troubleshooting

If users blocked:

check WAF logs
check rule priority
false positives

🚀 STEP 13 — ADD CLOUDWATCH + ALB LOGS

Why

SRE must see traffic

Enable ALB logs

EC2 → Load Balancer → Attributes → Enable Access Logs

Store in S3

Expected log

client_ip request_path target_status_code latency

Why important

You can debug:

500 errors
slow requests
bad clients

🚀 STEP 14 — ADD VPC FLOW LOGS

Already partially covered — now use it

Go to:

VPC → Flow Logs → Create

Example output

ACCEPT TCP 10.0.3.10 → 10.0.5.20 3306
REJECT TCP 1.2.3.4 → 10.0.5.20 3306

Why this matters

You can prove:

traffic allowed
traffic blocked

🚀 STEP 15 — ADD VPC ENDPOINT (PRIVATE AWS ACCESS)

Why

Private EC2 should NOT go through internet for AWS services

Go to:

VPC → Endpoints → Create

Service:

S3
Type: Gateway

Attach:

Private route table

Result

Private EC2 → S3 (no NAT, no internet)

SRE importance

secure
cheaper
required in enterprise

🚀 STEP 16 — ADD VPC PEERING (MULTI-VPC)

Scenario

You have:

VPC-A → your app
VPC-B → shared services

Create second VPC

CIDR:

10.1.0.0/16

Go to:

VPC → Peering → Create

Update routes BOTH SIDES

10.1.0.0/16 → peering
10.0.0.0/16 → peering

Result

Private communication between VPCs

SRE troubleshooting

routes missing?
SG blocking?
CIDR overlap?

🚀 STEP 17 — ADD TRANSIT GATEWAY (ENTERPRISE LEVEL)

Instead of many peerings:

Go to:

VPC → Transit Gateway → Create

Attach VPCs

Attach VPC-A
Attach VPC-B

Result

Central network hub

Why SRE uses this

scalable
cleaner architecture
used in large companies

🚀 STEP 18 — ADD PRIVATELINK (ADVANCED)

Scenario

Expose ONLY service, not full network

Flow

Consumer VPC → Endpoint → NLB → Service VPC

Why

secure
no full VPC access
SaaS architecture

Difference

Peering → full network
PrivateLink → one service

🚀 STEP 19 — ADD VPN (HYBRID CLOUD)

Scenario

Company has on-prem server

Go to:

VPC → VPN → Create Site-to-Site VPN

Result

On-prem → encrypted → AWS

SRE checks

tunnel UP?
routes correct?
firewall open?

🚀 STEP 20 — ADD DIRECT CONNECT (THEORY)

What

Private fiber connection

When used

banks
large companies

Difference

VPN → internet
Direct Connect → private line

🚀 STEP 21 — FINAL SRE TESTING (REAL SCENARIOS)

Scenario 1 — ALB down

Check:

DNS → OK?
ALB → Active?
Target → Healthy?

Scenario 2 — App slow

Check:

ALB logs
Latency
DB connection

Scenario 3 — DB not reachable

Check:

SG rules
Port 3306
Private routing

Scenario 4 — Private EC2 no internet

Check:

NAT
Route table
IGW

Scenario 5 — DNS issue

dig app.domain.com

🔥 WHAT YOU HAVE NOW (REAL SRE LEVEL)

You built:

✔ Multi-tier architecture
✔ DMZ design
✔ Private networking
✔ Load balancing
✔ Firewall (SG + WAF)
✔ DNS routing
✔ Observability (logs + flow logs)
✔ Private AWS access (VPC endpoint)
✔ Multi-VPC (peering + transit)
✔ Service exposure (PrivateLink)
✔ Hybrid cloud (VPN)

💬 FINAL INTERVIEW ANSWER

You say:

I built a production-grade AWS architecture with DNS using Route 53, public access through an Application Load Balancer in DMZ subnets, private application and database tiers, secure communication using security groups, outbound internet via NAT Gateway, private AWS access via VPC endpoints, and network observability using VPC Flow Logs and ALB logs. I also implemented multi-VPC connectivity using VPC peering and Transit Gateway, and secure service exposure using PrivateLink, along with hybrid connectivity using VPN.

Full SRE Networking Lecture: What You Must Know After Basic VPC

Aisalkyn Aidarova — Thu, 30 Apr 2026 00:00:32 +0000

1. DNS and Route 53

DNS is one of the most important networking topics for SRE. Many production outages look like “application is down,” but the real issue is DNS.

DNS translates a name into an IP address or another DNS name.

Example:

jumptotech.com → ALB DNS name → EC2 targets

In AWS, the main DNS service is Amazon Route 53. Route 53 is used to manage domain records and route users to AWS resources like ALB, CloudFront, S3 static websites, or failover endpoints.

Important DNS record types:

A record     → domain to IPv4 address
AAAA record  → domain to IPv6 address
CNAME        → domain to another domain
ALIAS        → Route 53 record pointing to AWS resources like ALB or CloudFront
TXT          → verification, SPF, DKIM, security records
MX           → email routing

In production, for an application, the flow is usually:

User
 ↓
Route 53
 ↓
Application Load Balancer
 ↓
Private application servers

As an SRE, when a website is not reachable, you should not immediately check EC2. First check DNS.

Use:

nslookup example.com
dig example.com
dig example.com +short

Troubleshooting questions:

Does the domain resolve?
Does it point to the correct ALB?
Was DNS changed recently?
Is TTL too long?
Is Route 53 health check failing?
Is the record public or private hosted zone?

Route 53 can also use health checks and failover routing. AWS recommends evaluating target health for alias records when using health-based DNS routing, otherwise Route 53 may still route traffic to unhealthy resources. (AWS Documentation)

Interview answer:

Route 53 is AWS DNS service. I use it to route user traffic to AWS resources such as ALB or CloudFront. As an SRE, I troubleshoot DNS by checking resolution, record type, TTL, hosted zone, and whether the DNS target is healthy.

2. Load Balancing: ALB vs NLB

A load balancer distributes traffic across multiple targets. In production, users should not directly access EC2 instances. They should access a load balancer.

Main AWS load balancers:

ALB = Application Load Balancer
NLB = Network Load Balancer

Application Load Balancer

ALB works at Layer 7, the application layer. It understands HTTP and HTTPS.

Use ALB for:

web applications
APIs
path-based routing
host-based routing
HTTPS termination
microservices
containerized apps

Example:

app.example.com      → frontend target group
api.example.com      → backend target group
/example/orders      → orders service
/example/payments    → payment service

ALB uses:

Listener
Rule
Target Group
Health Check

A listener receives traffic on a port, usually 80 or 443. A rule decides where to forward the request. A target group contains EC2, ECS tasks, IPs, or Lambda targets. Health checks decide whether the target should receive traffic. AWS documentation says ALB target groups route requests to registered targets and health checks are configured per target group. (AWS Documentation)

Production flow:

Internet
 ↓
Route 53
 ↓
ALB in public subnet
 ↓
EC2/ECS/EKS app in private subnet
 ↓
RDS database in private subnet

As an SRE, if ALB returns 503, usually it means no healthy targets.

Check:

Are targets registered?
Are targets healthy?
Is health check path correct?
Is app listening on correct port?
Does app security group allow traffic from ALB security group?
Is the app returning 200 on health check path?

Useful commands:

curl -I http://alb-dns-name
curl http://private-app-ip:8080/health

Network Load Balancer

NLB works at Layer 4. It handles TCP, UDP, and TLS traffic.

Use NLB for:

very high performance
TCP applications
static IP requirement
low latency
non-HTTP protocols

Examples:

Kafka
database proxy
game servers
TCP services

NLB health checks determine whether targets are available. AWS documentation says NLB uses active and passive health checks and routes traffic only to healthy targets in enabled Availability Zones unless cross-zone load balancing is enabled. (AWS Documentation)

Interview answer:

I use ALB for HTTP and HTTPS applications because it supports Layer 7 routing, TLS termination, host-based and path-based rules. I use NLB for high-performance TCP or UDP workloads where low latency or static IP is required.

3. Target Groups and Health Checks

Health checks are critical for reliability.

A load balancer should not send traffic to a broken server. That is why every target group has a health check.

Example health check:

Protocol: HTTP
Path: /health
Success code: 200
Interval: 30 seconds
Healthy threshold: 3
Unhealthy threshold: 2

Bad health check:

Why? Maybe homepage works but database connection is broken.

Better health check:

/health

This endpoint should check:

application running
database reachable
required dependencies available

But do not make health checks too heavy. If /health runs expensive database queries every few seconds, the health check itself can overload the app.

As an SRE, when deployment causes outage, check target group health first.

Troubleshooting:

Target unhealthy because timeout?
Target unhealthy because 403?
Target unhealthy because 500?
Wrong port?
Wrong path?
Security group blocking ALB?
App binding to localhost only?

Common mistake:

Application listens on:

127.0.0.1:8080

But it should listen on:

0.0.0.0:8080

Interview answer:

Health checks allow the load balancer to remove unhealthy targets from rotation. As an SRE, I always verify the health check path, port, response code, security group, and application logs.

4. VPC Endpoints

VPC endpoints allow private resources to access AWS services without using the public internet.

AWS documentation says VPC endpoints privately connect your VPC to supported AWS services without requiring an internet gateway, NAT device, VPN, or Direct Connect. Traffic stays on the AWS network backbone. (AWS Documentation)

Without VPC endpoint:

Private EC2
 ↓
NAT Gateway
 ↓
Internet path
 ↓
S3

With VPC endpoint:

Private EC2
 ↓
VPC Endpoint
 ↓
S3

Types:

Gateway Endpoint   → S3, DynamoDB
Interface Endpoint → SSM, ECR, CloudWatch, Secrets Manager, STS, KMS

Why SRE uses VPC endpoints:

increase security
reduce NAT Gateway dependency
reduce NAT data processing cost
allow private subnet access to AWS services
support private architecture

Very important real-world example:

You have private EC2 with no public IP. You want to connect using AWS Systems Manager Session Manager.

You may need interface endpoints for:

ssm
ssmmessages
ec2messages

If your private EC2 needs to pull Docker images from ECR, you may need endpoints for:

ecr.api
ecr.dkr
s3
CloudWatch Logs

Troubleshooting endpoint issues:

Is endpoint created in correct VPC?
Is private DNS enabled?
Is security group allowing HTTPS 443 to endpoint?
Is route table configured for gateway endpoint?
Does IAM policy allow access?
Is endpoint policy blocking access?

Interview answer:

I use VPC endpoints when private workloads need to access AWS services without going through the public internet or NAT Gateway. This improves security and can reduce cost.

5. AWS PrivateLink

PrivateLink is related to VPC endpoints, but it is more advanced.

AWS PrivateLink allows private connectivity between VPCs, AWS services, services in other AWS accounts, and Marketplace services without using public internet, NAT, VPN, or Direct Connect. (AWS Documentation)

Use case:

Company A exposes service privately.

Company B consumes it privately.

Consumer VPC
 ↓
Interface Endpoint
 ↓
PrivateLink
 ↓
Provider NLB
 ↓
Provider service

PrivateLink is service-level access, not full network access.

This is very important.

Difference:

VPC Peering     → connects networks
Transit Gateway → connects many networks
PrivateLink     → exposes only one service privately

Why this matters:

With VPC peering, VPCs can potentially route to many internal resources.

With PrivateLink, the consumer can access only the specific service exposed through the endpoint.

When to use PrivateLink:

SaaS provider exposing private API
shared internal service
cross-account service access
security-sensitive architecture
avoid full VPC-to-VPC routing

Interview answer:

PrivateLink is used when we want to expose a specific service privately without giving full network access between VPCs. It is more controlled than VPC peering.

6. VPC Peering

VPC peering connects two VPCs using private IPs.

Example:

VPC A: 10.0.0.0/16
VPC B: 10.1.0.0/16

After peering:

EC2 in VPC A → private IP → EC2 in VPC B

Rules:

CIDR blocks cannot overlap
Peering must be accepted
Routes must be added on both sides
Security groups must allow traffic
NACLs must allow traffic
Peering is not transitive

Not transitive means:

VPC A peers with VPC B
VPC B peers with VPC C
VPC A cannot automatically reach VPC C

Use peering when:

only two or a few VPCs need communication
simple architecture
low operational complexity

Do not use peering when you have many VPCs. It becomes hard to manage.

Troubleshooting peering:

Is peering active?
Are CIDRs overlapping?
Does VPC A route table point to peering connection?
Does VPC B route table point back?
Do SG/NACL allow traffic?
Is DNS resolution enabled if using private DNS names?

Interview answer:

VPC peering is private connectivity between two VPCs. It is simple and low-latency, but it does not support transitive routing and does not scale well for many VPCs.

7. Transit Gateway

Transit Gateway is like a cloud router.

Instead of creating many VPC peering connections, you attach VPCs to one central hub.

Without Transit Gateway:

VPC A ↔ VPC B
VPC A ↔ VPC C
VPC A ↔ VPC D
VPC B ↔ VPC C
...

This becomes messy.

With Transit Gateway:

VPC A
  ↓
Transit Gateway
  ↑
VPC B
  ↑
VPC C
  ↑
VPN / Direct Connect

Use Transit Gateway for:

many VPCs
multi-account architecture
shared services VPC
hybrid cloud
centralized firewall inspection
enterprise networks

AWS VPC connectivity documentation lists Transit Gateway, VPC peering, PrivateLink, VPN, and Direct Connect as major private connectivity options. (AWS Documentation)

As an SRE, you need to know that Transit Gateway has route tables too.

Troubleshooting Transit Gateway:

Is VPC attached to TGW?
Is attachment available?
Is route propagated?
Is route associated with correct TGW route table?
Do subnet route tables point to TGW?
Do SG/NACL allow traffic?
Is there asymmetric routing?

Interview answer:

Transit Gateway is used as a central network hub to connect many VPCs and hybrid networks. It is better than VPC peering when the environment has many VPCs or accounts.

8. VPN and Direct Connect

These are used for hybrid cloud: connecting on-premises data centers to AWS.

Site-to-Site VPN

VPN creates encrypted tunnels over the public internet.

Flow:

On-prem router/firewall
 ↓ encrypted tunnel
AWS VPN Gateway / Transit Gateway
 ↓
VPC

Use VPN when:

quick setup
lower cost
backup connection
encrypted connection over internet

Limitations:

internet-dependent
latency can vary
bandwidth limited compared to Direct Connect

Direct Connect

Direct Connect is a dedicated private network connection from your data center or colocation to AWS.

Use Direct Connect when:

stable latency required
large data transfer
enterprise hybrid cloud
more predictable performance

AWS documentation describes connectivity options using Direct Connect, Site-to-Site VPN, and Transit Gateway for remote network to VPC connectivity. (AWS Documentation)

Production design often uses:

Direct Connect as primary
VPN as backup
Transit Gateway as hub

Troubleshooting hybrid connectivity:

Is tunnel up?
Is BGP established?
Are routes advertised?
Are security groups allowing traffic?
Are on-prem firewalls allowing traffic?
Is return route correct?
Is DNS resolving private names?

Interview answer:

VPN provides encrypted connectivity over the internet, while Direct Connect provides a dedicated private connection to AWS. In production, companies often use Direct Connect for stable performance and VPN as backup.

9. AWS WAF

WAF means Web Application Firewall.

Security Group controls ports and IP access.

NACL controls subnet-level traffic.

WAF protects web applications at Layer 7.

WAF can block:

SQL injection
cross-site scripting
bad bots
malicious IPs
rate-based attacks
suspicious headers

Common placement:

User
 ↓
Route 53
 ↓
CloudFront or ALB
 ↓
WAF
 ↓
Application

Use WAF when:

public web application
API exposed to internet
compliance requirement
need Layer 7 protection
need rate limiting

Troubleshooting WAF:

Is WAF blocking legitimate traffic?
Check WAF logs
Check rule priority
Check managed rule false positives
Check rate limit
Check IP reputation list

Interview answer:

WAF protects applications at Layer 7 from web attacks such as SQL injection, XSS, and bad bots. I use it in front of ALB or CloudFront for internet-facing applications.

10. CloudFront and CDN Basics

CloudFront is AWS CDN.

CDN means content delivery network.

It caches content close to users.

Example:

Without CloudFront:

User in California → ALB in Virginia

With CloudFront:

User in California → nearest edge location → origin

Use CloudFront for:

static websites
images
videos
frontend apps
API acceleration
global users
DDoS protection with Shield
TLS termination

CloudFront origin can be:

S3
ALB
EC2
API Gateway
custom domain

SRE troubleshooting:

Is cache serving old content?
Is origin healthy?
Is behavior path correct?
Is HTTPS certificate valid?
Is WAF blocking request?
Is DNS pointing to CloudFront?

Common issue:

You deploy new frontend, but users still see old version.

Fix:

CloudFront invalidation

Interview answer:

CloudFront improves performance by caching content at edge locations closer to users. As an SRE, I troubleshoot CloudFront by checking cache behavior, origin health, invalidations, certificates, WAF, and DNS.

11. Network Observability

SRE must prove what is happening in the network.

Important tools:

VPC Flow Logs
ALB access logs
CloudWatch metrics
CloudTrail
Route 53 query logs
WAF logs
Transit Gateway flow logs

VPC Flow Logs

VPC Flow Logs capture IP traffic metadata for network interfaces.

They help answer:

Was traffic accepted or rejected?
Which source IP connected?
Which destination port?
Which ENI?
Which subnet?

Use VPC Flow Logs for:

security investigation
NACL troubleshooting
SG troubleshooting
network visibility
unexpected traffic analysis

Example:

REJECT TCP 10.0.3.10 10.0.5.20 5432

This tells you traffic to database port was rejected.

ALB access logs

ALB logs show:

client IP
request path
target status code
load balancer status code
response time
target processing time

Useful for:

502
503
504
slow requests
bad target responses

SRE book mindset

Google’s SRE book emphasizes that monitoring should help decide what should interrupt a human and what should not. Good monitoring is not collecting everything; good monitoring detects user-impacting issues. (Google SRE)

Interview answer:

For network observability, I use VPC Flow Logs, ALB logs, Route 53 logs, WAF logs, and CloudWatch metrics. These help me identify whether the issue is DNS, routing, firewall, load balancer, target health, or application.

12. Full Production Network Architecture

This is the architecture you must be able to explain in interviews.

Users
 ↓
Route 53
 ↓
CloudFront + WAF
 ↓
Application Load Balancer - public subnets
 ↓
Application servers / ECS / EKS - private app subnets
 ↓
RDS / ElastiCache - private DB subnets

Supporting components:

NAT Gateway       → private outbound internet
VPC Endpoint      → private AWS service access
Transit Gateway   → multi-VPC connectivity
VPN/DX            → on-prem connectivity
VPC Flow Logs     → network observability
CloudWatch        → metrics and alarms

Production rules:

ALB goes in public subnets
App goes in private subnets
DB goes in private subnets
NAT Gateway goes in public subnet
Private servers do not get public IPs
DB is never open to internet
Use SG references instead of hardcoded IPs
Use Multi-AZ for availability
Use VPC endpoints for private AWS service access
Use WAF for internet-facing apps

13. SRE Troubleshooting Framework

When production is down, do not guess.

Follow this order:

1. DNS
2. CDN / WAF
3. Load Balancer
4. Security Groups
5. Route Tables
6. NACL
7. Target Health
8. Application Logs
9. Database
10. Dependencies

Scenario 1: Website is down

Check:

dig app.example.com
curl -I https://app.example.com

Then:

Is Route 53 pointing to correct ALB/CloudFront?
Is certificate valid?
Is WAF blocking?
Is ALB reachable?
Are targets healthy?
Is app running?

Scenario 2: ALB returns 503

Meaning:

No healthy targets

Check:

Target group health
Health check path
Security group from ALB to app
App port
App logs
Deployment status

Scenario 3: ALB returns 504

Meaning:

Gateway timeout

Check:

App too slow?
DB slow?
Target not responding?
Timeout configuration?
Network path blocked?

Scenario 4: Private EC2 cannot access internet

Check:

Private route table has 0.0.0.0/0 → NAT Gateway
NAT Gateway is available
NAT Gateway has Elastic IP
NAT is in public subnet
Public subnet routes 0.0.0.0/0 → IGW
SG allows outbound
NACL allows ephemeral ports

Scenario 5: EC2 cannot access S3 privately

Check:

Gateway endpoint exists?
Route table associated?
Bucket policy allows endpoint?
IAM role allows S3?
Region correct?

Scenario 6: App cannot connect to RDS

Check:

RDS running?
Correct endpoint?
Correct port?
DB SG allows app SG?
App subnet route table has local route?
NACL allows traffic both directions?
Credentials correct?
DB max connections reached?

Scenario 7: VPC peering not working

Check:

Peering active?
CIDR non-overlapping?
Routes added both sides?
SG allows remote CIDR or SG?
NACL allows?
DNS resolution enabled?

14. What You Must Memorize for Interview

You must know this table:

Route 53        → DNS
CloudFront      → CDN / edge caching
WAF             → Layer 7 web protection
ALB             → HTTP/HTTPS load balancing
NLB             → TCP/UDP load balancing
VPC             → private network
Subnet          → network segment in one AZ
Route Table     → controls traffic direction
IGW             → internet access for public subnets
NAT Gateway     → outbound internet for private subnets
SG              → stateful resource firewall
NACL            → stateless subnet firewall
VPC Endpoint    → private access to AWS services
PrivateLink     → private service exposure
VPC Peering     → private VPC-to-VPC network connection
Transit Gateway → central router for many VPCs
VPN             → encrypted hybrid connection over internet
Direct Connect  → private dedicated hybrid connection
Flow Logs       → network traffic visibility

15. Strong Final Interview Answer

Use this answer:

I design AWS networking using layered architecture. I use Route 53 for DNS, CloudFront and WAF for edge performance and security, ALB for public application entry, private subnets for application workloads, and isolated private subnets for databases. I use NAT Gateway only for outbound internet from private subnets and VPC endpoints when private workloads need AWS service access without internet. For multi-VPC communication, I choose VPC peering for simple cases, Transit Gateway for large enterprise hub-and-spoke architecture, and PrivateLink when only one private service should be exposed. As an SRE, I troubleshoot from DNS to load balancer, route tables, security groups, NACLs, target health, logs, and application dependencies.

Observability, Reliability, and Incident Management (Production-Level)

Aisalkyn Aidarova — Wed, 29 Apr 2026 23:53:07 +0000

1. What SRE Actually Does (Real World)

After networking (VPC, subnets, routing), your system is running.

Now SRE responsibility starts:

Is the system working?
Is it fast?
Is it reliable?
Can we detect problems early?
Can we recover quickly?

This is called reliability engineering.

A simple way to think:

DevOps builds system
SRE keeps it alive under stress

2. Observability — Deep Explanation

Observability is not just “monitoring.”
Monitoring tells you something is wrong
Observability tells you why it is wrong

AWS and Google SRE books define observability as:

Ability to understand system state using external outputs

These outputs are:

metrics
logs
traces

2.1 Metrics (Deep)

Metrics are numerical time-series data

Examples:

CPU usage = 70%
requests/sec = 200
error rate = 5%

Why we use metrics

detect anomalies
trigger alerts
track performance trends
capacity planning

Tool: Amazon CloudWatch

What it does

collects metrics from AWS services (EC2, ALB, RDS)
stores time-series data
creates alarms

How we use it (real scenario)

Example:

You deploy application on EC2

CloudWatch automatically gives:

CPUUtilization
NetworkIn/Out
DiskReadOps

Then you create alarm:

IF CPU > 80% for 5 minutes → trigger alert

When to use CloudWatch

AWS native monitoring
quick setup
infrastructure-level metrics

Limitations (SRE thinking)

not very strong for custom application metrics
limited visualization compared to Prometheus + Grafana

2.2 Metrics Tool (Advanced): Prometheus

What it does

pulls metrics from applications
stores time-series data
supports powerful queries (PromQL)

Why SRE prefers Prometheus

better for microservices
supports custom metrics
integrates with Kubernetes

How we use it

Application exposes metrics endpoint:
/metrics
Prometheus scrapes it
You query:

request latency
error rates
DB connections

Example

You detect:

high latency
normal CPU

→ issue is NOT infrastructure
→ issue is application

Troubleshooting using metrics

Case:

Website slow

Check:

CPU high → scaling issue
latency high → app issue
error rate high → bug or DB problem

2.3 Logs (Deep)

Logs are detailed events

Example:

user login failed
DB connection error
API returned 500

Tool: ELK Stack

Components:

Elasticsearch → storage
Logstash → processing
Kibana → visualization

Why logs are critical

Metrics say:
“Error rate increased”

Logs say:
“Database connection timeout”

How we use logs

Good logs must include:

timestamp
service name
log level (INFO, ERROR)
request ID

AWS logging: CloudWatch Logs

collects logs from EC2, Lambda
integrates with CloudWatch metrics

Troubleshooting using logs

Case:

App returns 500

Steps:

check logs
find error message
identify root cause

Example:

“connection refused” → DB issue
“timeout” → network issue

2.4 Tracing (Deep)

Tracing tracks request across services

Example:

User request path:

User → ALB → API → Service → DB

Tool: AWS X-Ray

Why tracing matters

In microservices:

You don’t know where latency happens

Tracing shows:

which service is slow
where failure occurs

Example

Request takes 3 seconds

Tracing shows:

API: 50ms
Service: 100ms
DB: 2.8s

→ problem is DB

3. SLI, SLO, SLA (Deep Understanding)

SLI (Indicator)

What you measure:

uptime
latency
error rate

SLO (Objective)

Target:

99.9% uptime

SLA (Agreement)

Legal commitment:

if broken → compensation

Why SRE uses this

Because:

Without SLO → no reliability target
Without SLI → no measurement

4. Alerting (Real SRE Thinking)

Alerting is where most teams fail

Bad alerts

CPU 80%
disk usage 70%

These create noise

Good alerts

user cannot login
API error rate > 5%
latency > threshold

Tool: Prometheus Alertmanager

How alert works

metric collected
condition evaluated
alert fired
notification sent (Slack, email)

SRE rule

Alert on user impact, not infrastructure

5. Incident Management (Production Flow)

Incident = service disruption

Real steps

detection (monitoring)
alert triggered
engineer responds
mitigation (temporary fix)
resolution (root cause fix)
postmortem

Example

Issue:
Website down

Actions:

restart service (mitigation)
fix DB connection (resolution)

6. Postmortem (Critical SRE Practice)

After incident:

You must answer:

what happened?
why?
how to prevent?

Rule

No blame

Focus on system failure, not people

7. Error Budget (Advanced Concept)

If SLO = 99.9%

Allowed downtime:

≈ 43 minutes/month

Why important

Balance:

innovation (deploy fast)
stability (avoid downtime)

8. High Availability (HA)

System must survive failure

AWS tools

Elastic Load Balancer
Multi-AZ deployment

Example

If one AZ fails:

Traffic shifts to another AZ

9. Auto Scaling (Reliability + Cost)

Automatically adjust capacity

AWS service

Auto Scaling Group

Example

Traffic spike:

add EC2 instances

Traffic drop:

remove instances

10. Health Checks

Check system status

In Kubernetes

readiness probe → ready to serve
liveness probe → alive

Tool: Kubernetes

Why important

Without health checks:

Load balancer sends traffic to broken app

11. Caching (Performance)

Store frequently used data

Tool: Redis

Why use caching

reduce DB load
faster response

12. Disaster Recovery

Plan for failure

Strategies

backup restore
multi-region
active-active

13. Troubleshooting Mindset (MOST IMPORTANT)

When something breaks:

DO NOT GUESS

Follow layers:

DNS
Network
Load balancer
App
DB

Example

App not working

Check:

DNS resolves?
ALB healthy?
EC2 running?
logs show error?
DB reachable?

FINAL SRE INTERVIEW ANSWER

You say:

As an SRE, I focus on system reliability by implementing observability using metrics, logs, and traces, defining SLOs, setting up meaningful alerts, ensuring high availability with load balancing and auto scaling, and handling incidents with structured troubleshooting and postmortems.

VPC, subnets, IGW, NAT, routing, firewall, DMZ, private DB, and troubleshooting part #1

Aisalkyn Aidarova — Wed, 29 Apr 2026 01:15:42 +0000

🔥 LAB GOAL (PRODUCTION STYLE)

You will build:

Internet
   ↓
Load Balancer (DMZ / Public)
   ↓
Web Server (Private)
   ↓
Database (Private)

With:

Public subnets (DMZ)
Private subnets (App + DB)
NAT Gateway
Security Groups (firewall)
Route tables (routing)

🚀 STEP 0 — WHAT YOU MUST HAVE

Already created:

✔ VPC
✔ 2 Public subnets
✔ 2 Private subnets
✔ Internet Gateway
✔ NAT Gateway

🚀 STEP 1 — FIX ROUTING (VERY IMPORTANT)

Public Route Table

Go to VPC → Route Tables → Public RT

Make sure:

0.0.0.0/0 → Internet Gateway

Associate:

Public Subnet 1
Public Subnet 2

Private Route Table

Make sure:

0.0.0.0/0 → NAT Gateway

Associate:

Private Subnet 1
Private Subnet 2

✔ Result:

Public = internet access
Private = outbound only

🚀 STEP 2 — CREATE SECURITY GROUPS (FIREWALL DESIGN)

1. Load Balancer SG (`alb-sg`)

Allow:

HTTP 80 → 0.0.0.0/0

2. Web Server SG (`web-sg`)

Allow:

HTTP 80 → alb-sg
SSH 22 → your IP

3. Database SG (`db-sg`)

Allow:

MySQL 3306 → web-sg

✔ Result:

Internet → only ALB
ALB → Web
Web → DB
Users CANNOT access DB

👉 This is real firewall architecture

🚀 STEP 3 — CREATE LOAD BALANCER (DMZ)

Use:
Application Load Balancer

Where:

EC2 → Load Balancers → Create

Config:

Type: Application LB
Scheme: Internet-facing
Subnets:
- Public Subnet 1
- Public Subnet 2
Security Group:
- alb-sg

✔ Result:

👉 Entry point for users

🚀 STEP 4 — CREATE WEB SERVERS (PRIVATE)

Launch 2 EC2:

Subnet:
- private-subnet-1
- private-subnet-2
Security Group:
- web-sg
NO public IP

Install nginx:

sudo apt update
sudo apt install nginx -y

Customize page:

echo "Hello from Web Server 1" | sudo tee /var/www/html/index.html

✔ Result:

👉 Private app servers running

🚀 STEP 5 — CONNECT ALB → WEB

Create Target Group:

Type: Instance
Port: 80

Add both EC2 instances

Attach to Load Balancer

✔ Result:

👉 ALB sends traffic to web servers

🚀 STEP 6 — TEST

Open:

http://<ALB-DNS>

✔ Result:

👉 You see your web page

Refresh:
👉 It switches between servers

🚀 STEP 7 — CREATE DATABASE (SIMULATION)

You can use EC2 or:
Amazon RDS

For simple lab (EC2 DB):

Launch EC2:

Subnet: private-subnet-1
SG: db-sg

✔ Result:

👉 Private DB server

🚀 STEP 8 — TEST NETWORK SECURITY

Try:

From your laptop:

Access DB → ❌ FAIL

From web EC2:

Connect DB → ✔ WORK

👉 This proves firewall working

🚀 STEP 9 — TEST NAT (VERY IMPORTANT)

SSH into web EC2:

ping google.com

✔ Result:

👉 Works → NAT is correct

🚀 STEP 10 — BREAK & DEBUG (SRE LEVEL)

Now simulate failures:

Scenario 1 — Remove NAT route

👉 Private EC2 cannot reach internet

Fix:
👉 Add NAT route back

Scenario 2 — Remove SG rule (web → db)

👉 App cannot reach DB

Fix:
👉 Add rule back

Scenario 3 — Stop one EC2

👉 App still works via ALB

👉 This is real SRE behavior

🔥 WHAT YOU JUST LEARNED

You implemented:

✔ VPC design
✔ Subnet segmentation (DMZ / Private)
✔ Routing (IGW + NAT)
✔ Firewall (SG)
✔ Load balancing
✔ Secure DB access
✔ Failure testing

💬 INTERVIEW ANSWER

I built a multi-tier architecture in AWS with public and private subnets, configured routing using Internet Gateway and NAT Gateway, secured communication using security groups, deployed web servers behind an Application Load Balancer, and validated failover and connectivity through testing scenarios.

AWS Networking Full Lecture for DevOps/SRE

Aisalkyn Aidarova — Wed, 29 Apr 2026 01:12:48 +0000

AWS networking starts with one main idea: we need to build a secure private network where some resources are public, some are private, traffic flows correctly, and we can troubleshoot when something breaks.

In traditional networking, you used switches, routers, VLANs, firewalls, and DMZ. In AWS, the same ideas exist, but they are software-defined. Instead of physical routers and switches, we use VPC, subnets, route tables, internet gateway, NAT gateway, security groups, network ACLs, VPC endpoints, and VPC peering.

1. VPC

A VPC, or Virtual Private Cloud, is your private network inside AWS. It is logically isolated from other customers. When you create a VPC, you choose a CIDR block, for example:

10.0.0.0/16

This means your AWS network has private IP addresses from the 10.0.x.x range.

Think of VPC like your company building. Inside that building, you create rooms. Those rooms are subnets.

We use VPC because we need control over:

IP ranges
subnets
routing
security
internet access
private communication

In interviews, say:

A VPC is a logically isolated network in AWS where we define IP ranges, subnets, routing, and security rules.

AWS route tables control where traffic goes inside the VPC, and each subnet must be associated with a route table. (AWS Documentation)

2. Subnet

A subnet is a smaller network inside the VPC.

Example:

VPC: 10.0.0.0/16

Public subnet 1:  10.0.1.0/24
Public subnet 2:  10.0.2.0/24
Private subnet 1: 10.0.3.0/24
Private subnet 2: 10.0.4.0/24

A subnet belongs to one Availability Zone.

We create multiple subnets because we want separation and high availability.

Public subnet is for resources that need internet access, like:

Application Load Balancer
Bastion host
NAT Gateway
Public web server for testing

Private subnet is for resources that should not be directly accessible from the internet, like:

Application servers
Databases
Internal APIs
Backend services

This is the AWS version of your Packet Tracer segmentation.

Packet Tracer VLAN = AWS subnet.

3. Public Subnet vs Private Subnet

A subnet is not automatically public or private because of its name. It becomes public or private based on its route table.

A public subnet has this route:

0.0.0.0/0 → Internet Gateway

A private subnet does not route directly to the Internet Gateway. Usually it has:

0.0.0.0/0 → NAT Gateway

So the real difference is routing.

Public subnet means resources can communicate with the internet if they also have a public IP.

Private subnet means resources cannot be reached directly from the internet.

4. Route Table

A route table is like the traffic controller for your VPC. AWS documentation describes it as rules that determine where traffic from your subnet or gateway is directed. (AWS Documentation)

Example public route table:

10.0.0.0/16 → local
0.0.0.0/0  → Internet Gateway

The local route allows resources inside the VPC to communicate with each other.

The 0.0.0.0/0 route means all unknown traffic, usually internet traffic, goes to the Internet Gateway.

Example private route table:

10.0.0.0/16 → local
0.0.0.0/0  → NAT Gateway

This means private servers can talk inside the VPC and can go out to the internet through NAT, but the internet cannot start a connection back to them.

Troubleshooting route tables:

If public EC2 is not accessible, check:

Does the subnet route table have 0.0.0.0/0 → Internet Gateway?
Does the EC2 have public IP?
Does the security group allow traffic?
Is the instance running?

If private EC2 cannot reach the internet, check:

Does private route table have 0.0.0.0/0 → NAT Gateway?
Is NAT Gateway available?
Is NAT Gateway in public subnet?
Does public subnet have route to Internet Gateway?

5. Internet Gateway

An Internet Gateway allows communication between your VPC and the internet.

But just attaching an Internet Gateway is not enough. You must also update the public route table:

0.0.0.0/0 → Internet Gateway

AWS documentation says public subnet route tables can use Internet Gateway as the target for traffic going to destinations not explicitly known, such as 0.0.0.0/0. (AWS Documentation)

We use Internet Gateway when we want public resources, such as:

Load balancer
Public web server
Bastion host
NAT Gateway

Correct design:

Internet
   ↓
Internet Gateway
   ↓
Public Route Table
   ↓
Public Subnet

Common issue:

People create an Internet Gateway but forget to associate the public subnet with the public route table. Then EC2 will not be reachable.

6. NAT Gateway

NAT Gateway is used for private subnet resources that need outbound internet access but should not be reachable from the internet.

Example:

Your private EC2 needs to run:

sudo apt update
sudo apt install nginx
docker pull image

It needs internet access. But you do not want the internet to SSH into it.

That is why we use NAT Gateway.

AWS describes NAT Gateway as a NAT service that lets instances in private subnets connect to services outside the VPC while external services cannot initiate connections to those private instances. (AWS Documentation)

Correct NAT design:

Private EC2
   ↓
Private Route Table
   ↓
NAT Gateway in Public Subnet
   ↓
Internet Gateway
   ↓
Internet

Important rule:

NAT Gateway must be in a public subnet.

Why?

Because NAT Gateway itself needs internet access through Internet Gateway.

Private route table should have:

0.0.0.0/0 → NAT Gateway

Public route table should have:

0.0.0.0/0 → Internet Gateway

NAT Gateway is zonal. For production, best practice is one NAT Gateway per Availability Zone. If you have private subnet in AZ-a and private subnet in AZ-b, each private subnet should use NAT in the same AZ for reliability and to avoid cross-AZ dependency.

Troubleshooting NAT:

If private EC2 cannot access internet, check:

Is NAT Gateway available?
Is NAT Gateway in public subnet?
Does NAT Gateway have Elastic IP?
Does public subnet route to Internet Gateway?
Does private subnet route to NAT Gateway?
Does security group allow outbound traffic?
Does NACL allow inbound/outbound ephemeral ports?

7. Elastic IP

Elastic IP is a static public IPv4 address in AWS.

We use Elastic IP when we need a fixed public IP that does not change.

NAT Gateway requires Elastic IP because private instances going out to the internet need a stable public source IP.

Example:

Private EC2 → NAT Gateway → Internet

From the internet side, traffic appears to come from the NAT Gateway Elastic IP.

Use Elastic IP for:

NAT Gateway
Bastion host
Static public server
Allowlisting with external vendors

Do not overuse Elastic IP. In production, most public application traffic should go through a Load Balancer, not directly to EC2.

8. Firewall

A firewall controls traffic based on rules.

In AWS, firewall behavior mainly comes from:

Security Groups
Network ACLs
AWS Network Firewall

For most normal EC2-level access, you use security groups.

A firewall answers:

Who can connect?
From where?
To which port?
Using which protocol?

Example:

Web server security group:

Inbound:
HTTP 80 from 0.0.0.0/0
HTTPS 443 from 0.0.0.0/0
SSH 22 from my IP only

Database security group:

Inbound:
PostgreSQL 5432 only from app server security group

This is production thinking. Users should never directly access the database.

9. Security Group

Security Group is an instance-level firewall. It is attached to EC2, RDS, Load Balancer, and other resources.

Security Groups are stateful.

Stateful means if inbound traffic is allowed, return traffic is automatically allowed.

Example:

If user connects to web server on port 80, the response is automatically allowed back.

AWS explains that security groups control inbound and outbound traffic at the instance level. (AWS Documentation)

Good security group design:

Load Balancer SG:

Inbound:
80/443 from internet

Outbound:
To web/app servers

Web/App Server SG:

Inbound:
App port only from Load Balancer SG
SSH only from bastion or SSM

Outbound:
To database or internet through NAT

Database SG:

Inbound:
DB port only from App Server SG

Outbound:
Default or restricted depending on company policy

Very important SRE idea:

Use security group references instead of IP addresses when possible.

Example:

Instead of:

Allow 10.0.3.10 on port 5432

Use:

Allow app-server-sg on port 5432

This is better because EC2 IPs can change.

10. Network ACL

Network ACL, or NACL, is subnet-level firewall.

Security Group protects the instance.
NACL protects the subnet.

AWS says Network ACLs control traffic in and out of one or more subnets and can be used as an additional layer of security. (AWS Documentation)

NACL is stateless.

Stateless means you must allow both inbound and outbound traffic separately.

Example:

If inbound HTTP is allowed but outbound response traffic is denied, connection fails.

NACL rules have numbers:

100 allow HTTP
110 allow HTTPS
120 deny specific IP
* deny all

Lower number is evaluated first.

When to use NACL:

Use NACL for broad subnet-level guardrails, for example:

Block known malicious IP
Block traffic between subnet groups
Add extra compliance layer

Do not use NACL for every small application rule. Use Security Groups for that.

AWS also recommends security groups as the primary network control and NACLs as optional stateless subnet-level guardrails. (AWS Documentation)

11. Security Group vs NACL

Security Group:

Instance level
Stateful
Only allow rules
Commonly used
Can reference another security group

NACL:

Subnet level
Stateless
Allow and deny rules
Rule number order matters
Used for broad subnet control

Interview answer:

Security Groups are stateful firewalls attached to resources, while NACLs are stateless firewalls applied at the subnet level.

12. DMZ

DMZ means demilitarized zone.

In networking, DMZ is the public-facing zone that sits between the internet and the private internal network.

In AWS, a DMZ is usually your public subnet.

DMZ contains:

Application Load Balancer
Bastion host
NAT Gateway
Sometimes public web servers

Private resources like databases should not be in DMZ.

Correct design:

Internet
   ↓
Public Subnet / DMZ
   ↓
Private App Subnet
   ↓
Private DB Subnet

Why we use DMZ:

Because public-facing components need controlled exposure, but internal systems must stay private.

Example:

User can reach:

User → ALB

ALB can reach:

ALB → App Server

App server can reach:

App Server → Database

User cannot reach:

User → Database

That is production security.

13. Proxy

A proxy is an intermediate server that forwards traffic.

There are two common types:

Forward proxy:

User → Proxy → Internet

Used when internal users access the internet through a controlled server.

Reverse proxy:

User → Reverse Proxy → Backend Servers

Used when external users access internal applications through a front layer.

In real DevOps/SRE, common reverse proxies are:

Nginx
HAProxy
Application Load Balancer
API Gateway
Ingress Controller in Kubernetes

Why use proxy:

Hide backend servers
Terminate SSL/TLS
Route traffic
Apply access control
Load balance
Log requests
Protect backend

Example:

User does not directly access app server.

Instead:

User → ALB/Nginx → App Server

That is reverse proxy behavior.

Firewall vs Proxy:

Firewall decides allow or deny.

Proxy receives and forwards application traffic.

14. VPC Endpoint

You said “endpoint servers.” In AWS, the important concept is VPC Endpoint.

A VPC Endpoint allows private resources to access AWS services without going through the public internet.

Example:

Private EC2 needs to access S3.

Without endpoint:

Private EC2 → NAT Gateway → Internet path → S3

With VPC endpoint:

Private EC2 → VPC Endpoint → S3

This is more secure and can reduce NAT traffic cost.

Types:

Gateway Endpoint:

S3
DynamoDB

Interface Endpoint:

SSM
CloudWatch
ECR
Secrets Manager
STS
many AWS services

When to use VPC endpoints:

Private subnet needs AWS service access
You want to avoid internet path
You want better security
You want to reduce NAT dependency

Example SRE use case:

Private EC2 has no public IP. You want to connect using SSM Session Manager. Then you may need interface endpoints for:

ssm
ssmmessages
ec2messages

15. VPC Peering

VPC Peering connects two VPCs privately.

Example:

VPC A: 10.0.0.0/16
VPC B: 10.1.0.0/16

After peering, instances in VPC A can communicate with instances in VPC B using private IPs.

Use VPC peering when:

Two applications are in different VPCs
Shared services VPC needs to talk to app VPC
Company has dev/prod/shared networks

Important rules:

CIDR blocks cannot overlap.

If VPC A and VPC B both use:

10.0.0.0/16

Peering will not work.

Also, VPC peering is not transitive.

If:

VPC A peers with VPC B
VPC B peers with VPC C

That does not mean A can talk to C automatically.

For many VPCs, companies often use Transit Gateway instead of many peering connections.

16. Routing Servers

In AWS, you usually do not manage a routing server like in traditional networking. AWS provides an implicit router inside the VPC. You control routing using route tables.

AWS documentation says a VPC has an implicit router, and route tables control where traffic is directed. (AWS Documentation)

However, sometimes companies deploy routing or network appliances, such as:

firewall appliance
proxy appliance
NAT instance
VPN router
inspection appliance

But for normal DevOps/SRE learning, focus first on:

Route tables
Internet Gateway
NAT Gateway
Transit Gateway
VPC Peering
VPC Endpoints

17. How to Build Correct AWS Network Architecture

For your lab, build this:

VPC: 10.0.0.0/16

Public Subnet 1: 10.0.1.0/24
Public Subnet 2: 10.0.2.0/24

Private App Subnet 1: 10.0.3.0/24
Private App Subnet 2: 10.0.4.0/24

Private DB Subnet 1: 10.0.5.0/24
Private DB Subnet 2: 10.0.6.0/24

Public route table:

10.0.0.0/16 → local
0.0.0.0/0  → Internet Gateway

Private route table:

10.0.0.0/16 → local
0.0.0.0/0  → NAT Gateway

Database subnet route table:

10.0.0.0/16 → local

For stricter security, database subnet does not need internet access.

Architecture:

Internet
   ↓
Internet Gateway
   ↓
Application Load Balancer in Public Subnets
   ↓
App EC2 in Private Subnets
   ↓
RDS Database in Private DB Subnets

This is real production design.

AWS also recommends using multiple Availability Zones for production applications because it improves high availability, fault tolerance, and scalability. (AWS Documentation)

18. How to Create It in Console

First create VPC:

VPC → Create VPC
Name: prod-vpc
CIDR: 10.0.0.0/16

Create subnets:

public-subnet-1   10.0.1.0/24  AZ-a
public-subnet-2   10.0.2.0/24  AZ-b
private-subnet-1  10.0.3.0/24  AZ-a
private-subnet-2  10.0.4.0/24  AZ-b

Create Internet Gateway:

VPC → Internet Gateways → Create
Attach to prod-vpc

Create public route table:

Route Tables → Create
Name: public-rt
Route: 0.0.0.0/0 → Internet Gateway
Associate public subnets

Create NAT Gateway:

VPC → NAT Gateways → Create
Subnet: public-subnet-1
Elastic IP: allocate

Create private route table:

Route Tables → Create
Name: private-rt
Route: 0.0.0.0/0 → NAT Gateway
Associate private subnets

Create EC2 in public subnet:

Auto public IP: enabled
Security Group: allow SSH from your IP, HTTP from internet

Create EC2 in private subnet:

No public IP
Security Group: allow SSH only from public/bastion SG or use SSM

19. Correct Troubleshooting Method

When something does not work, do not guess. Follow layers.

Problem: Public EC2 not reachable

Check:

1. Is EC2 running?
2. Does EC2 have public IP?
3. Is subnet associated with public route table?
4. Does public route table have 0.0.0.0/0 → IGW?
5. Is IGW attached to VPC?
6. Does security group allow inbound port?
7. Does NACL allow traffic?
8. Is OS firewall blocking traffic?
9. Is application running?

Problem: Private EC2 cannot reach internet

Check:

1. Is private subnet associated with private route table?
2. Does route table have 0.0.0.0/0 → NAT Gateway?
3. Is NAT Gateway available?
4. Is NAT Gateway in public subnet?
5. Does NAT Gateway have Elastic IP?
6. Does public subnet route to IGW?
7. Does security group allow outbound?
8. Does NACL allow outbound and return traffic?

Problem: App cannot connect to database

Check:

1. Is DB running?
2. Is DB in private subnet?
3. Does DB security group allow app SG?
4. Is correct DB port open?
5. Are app and DB in same VPC or connected VPCs?
6. Is route table allowing local VPC traffic?
7. Is DNS name correct?
8. Are credentials correct?

Problem: VPC Peering not working

Check:

1. Are CIDRs non-overlapping?
2. Is peering connection accepted?
3. Does VPC A route table point to peering connection?
4. Does VPC B route table point back?
5. Do security groups allow traffic?
6. Do NACLs allow traffic?

20. SRE Mindset

An SRE does not only create VPC. An SRE proves that the network is reliable.

That means you must test:

Can public users reach only what they should reach?
Can private servers reach internet through NAT?
Can database stay private?
Can one AZ fail and application still work?
Can logs show denied traffic?
Can we troubleshoot quickly?

For observability, enable:

VPC Flow Logs
CloudWatch metrics
ALB access logs
Security group review
NACL review

AWS VPC Flow Logs capture information about IP traffic going to and from network interfaces and can help diagnose security group and NACL problems. (AWS Documentation)

Final Interview Summary

You can say:

I design AWS networks using VPCs, public and private subnets, route tables, Internet Gateway, NAT Gateway, security groups, and NACLs. Public subnets are used for internet-facing components like load balancers, while private subnets host application and database resources. NAT Gateway allows private resources to access the internet without being exposed. Security Groups protect resources at the instance level, and NACLs provide subnet-level control. For private AWS service access, I use VPC endpoints, and for private communication between VPCs, I use VPC peering or Transit Gateway depending on scale.

proxy, firewall and DMZ on packet tracer

Aisalkyn Aidarova — Mon, 27 Apr 2026 13:33:47 +0000

PHASE 1 — CHECK EXISTING ARCHITECTURE FIRST

1. ROUTER0 CHECK COMMANDS

Run on Router0.

Check all router interfaces

enable
show ip interface brief

Expected:

g0/0.10    192.168.10.1    up/up
g0/0.20    192.168.20.1    up/up
g0/0.30    192.168.30.1    up/up
g0/1.50    192.168.50.1    up/up
g0/1.100   200.1.1.1       up/up

This shows:

Which VLAN gateway exists on router
Whether interfaces are working

Check routing table

show ip route

Expected:

192.168.10.0/24 connected
192.168.20.0/24 connected
192.168.30.0/24 connected
192.168.50.0/24 connected
200.1.1.0/24 connected

This shows:

Router knows all VLAN networks

Check DHCP pools on router

show running-config | section dhcp

Expected:

VLAN10 pool
VLAN20 pool
VLAN30 pool

This shows:

Router or server DHCP settings

Check firewall rules

show access-lists

Expected before firewall:

No important ACL or old ACL only

This shows:

Existing firewall rules

Check where ACL is applied

show running-config | include access-group

Expected before firewall:

empty or old access-group

This shows:

Whether firewall is already applied to interface

Check router full config

show running-config

This shows everything:

Subinterfaces
DHCP
ACL
NAT
Gateway IPs

2. SWITCH0 CHECK COMMANDS

Switch0 = user computers.

Run on Switch0.

Check VLAN and ports

enable
show vlan brief

Expected from your lab:

VLAN 10 HR       Fa0/1, Fa0/2, Fa0/5
VLAN 20 IT       Fa0/3
VLAN 30 DevOps   Fa0/4

This shows:

Which computer port belongs to which VLAN

Check trunk port

show interfaces trunk

Expected:

Fa0/24 trunking
Allowed VLANs: 10,20,30,50

This shows:

Switch-to-router connection carries VLANs

Check MAC address table

show mac address-table

Expected:

MAC addresses learned on PC ports and trunk port

This shows:

Which device is connected to which switch port

Check switch interfaces

show ip interface brief

Expected:

Fa0/1 up
Fa0/2 up
Fa0/3 up
Fa0/4 up
Fa0/24 up

This shows:

Which physical cables are active

3. SWITCH1 CHECK COMMANDS

Switch1 = servers.

Run on Switch1.

Check VLAN and server ports

enable
show vlan brief

Expected:

VLAN 50 SERVERS/PUBLIC   Fa0/1
VLAN 100 PUBLIC          Fa0/2, Fa0/4

Fa0/1 = Server0
Fa0/2 = Server2
Fa0/4 = PC4 or future server
Fa0/3 = trunk to router

Check trunk

show interfaces trunk

Expected:

Fa0/3 trunking
Allowed VLANs: 50,100

Check MAC address table

show mac address-table

Expected:

VLAN 50 MAC on Fa0/1
VLAN 100 MAC on Fa0/2
VLAN 100 MAC on Fa0/4

Check interface status

show ip interface brief

Expected:

Fa0/1 up
Fa0/2 up
Fa0/3 up
Fa0/4 up

4. COMPUTER CHECK COMMANDS

On every PC:

Desktop → Command Prompt

Run:

ipconfig

Expected:

PC0  = 192.168.10.10 / gateway 192.168.10.1
PC10 = 192.168.10.11 / gateway 192.168.10.1
PC1  = 192.168.10.12 / gateway 192.168.10.1
PC2  = 192.168.20.10 / gateway 192.168.20.1

Then test gateway:

ping 192.168.10.1

or for VLAN20:

ping 192.168.20.1

Expected:

Success

5. SERVER CHECKS

Server0 — DHCP/DNS server

Go to:

Server0 → Desktop → IP Configuration

Expected:

IP: 192.168.50.10
Mask: 255.255.255.0
Gateway: 192.168.50.1
DNS: 192.168.50.10

Services:

Services → DHCP → ON
Services → DNS → ON

Server2 — Public-facing web/proxy server

Expected:

IP: 200.1.1.2
Mask: 255.255.255.0
Gateway: 200.1.1.1
DNS: 192.168.50.10
DHCP Service: OFF
HTTP Service: ON

PHASE 2 — IMPLEMENT DMZ + FIREWALL + PROXY

Important:

Right now VLAN100 is only a separate network.
It becomes a DMZ after we apply firewall rules.

Production-style zones

INTERNAL USERS:
VLAN 10,20,30
192.168.10.0/24
192.168.20.0/24
192.168.30.0/24

PRIVATE SERVERS:
VLAN 50
192.168.50.0/24

DMZ / PUBLIC-FACING:
VLAN 100
200.1.1.0/24

STEP 1 — FIX SERVER2

On Server2:

Services → DHCP → OFF
Services → HTTP → ON

IP config:

IP: 200.1.1.2
Mask: 255.255.255.0
Gateway: 200.1.1.1
DNS: 192.168.50.10

STEP 2 — CREATE FIREWALL ON ROUTER

This firewall will do:

Internal users can access web/proxy server
DMZ cannot access internal PCs
DMZ cannot access private servers except database later
Users cannot directly access database later

Run on Router0:

enable
conf t

no access-list 100
no access-list 101
no access-list 110

ip access-list extended DMZ_FIREWALL
remark Allow DMZ server to reach private database later
permit tcp host 200.1.1.2 host 192.168.50.20 eq 80

remark Block DMZ from reaching internal user VLANs
deny ip 200.1.1.0 0.0.0.255 192.168.10.0 0.0.0.255
deny ip 200.1.1.0 0.0.0.255 192.168.20.0 0.0.0.255
deny ip 200.1.1.0 0.0.0.255 192.168.30.0 0.0.0.255

remark Block DMZ from reaching private server VLAN except database rule above
deny ip 200.1.1.0 0.0.0.255 192.168.50.0 0.0.0.255

remark Allow remaining traffic for Packet Tracer lab stability
permit ip any any

interface g0/1.100
ip access-group DMZ_FIREWALL in

end
wr

What this means

interface g0/1.100 = VLAN100 gateway
ip access-group DMZ_FIREWALL in = check traffic coming FROM DMZ into router

So when Server2 tries to go inside:

Server2 → Router → Internal network

Router checks firewall.

STEP 3 — INTERNAL USERS FIREWALL

This blocks users from directly accessing database later.

Run on Router0:

enable
conf t

ip access-list extended INTERNAL_USERS
remark Allow users to access DMZ web/proxy server
permit tcp 192.168.0.0 0.0.255.255 host 200.1.1.2 eq 80

remark Allow users to use DNS
permit udp 192.168.0.0 0.0.255.255 host 192.168.50.10 eq 53

remark Block users from accessing private database directly
deny tcp 192.168.0.0 0.0.255.255 host 192.168.50.20 eq 80

remark Allow other traffic for lab testing
permit ip any any

interface g0/0.10
ip access-group INTERNAL_USERS in

interface g0/0.20
ip access-group INTERNAL_USERS in

interface g0/0.30
ip access-group INTERNAL_USERS in

end
wr

🔥 PHASE 3 — CREATE INTERNET-FACING WEB APP

Step 1 — Create Website on Server2 (DMZ)

Go to:

Server2 → Services → HTTP → ON

Then edit index.html

Replace with:

<html>
<head>
<title>Company Portal</title>
</head>

<body>
<h1>Welcome to JumpToTech Company</h1>

<h2>Login</h2>

<form>
Username: <input type="text"><br><br>
Password: <input type="password"><br><br>
<input type="submit" value="Login">
</form>

</body>
</html>

Step 2 — TEST WEB SERVER

From any PC (PC0):

ping 200.1.1.2

Expected:

Success

Now open browser:

Desktop → Web Browser
http://200.1.1.2

✔️ You should see your webpage

🔥 PHASE 4 — CREATE PRIVATE DATABASE

We simulate database using another server.

Step 1 — Use Server0 or New Server as DB

👉 Better: use Server0 as DB + DNS

Assign (already done):

IP: 192.168.50.10

Step 2 — Create Database (simulate via HTTP)

Go to:

Server0 → Services → HTTP → ON

Edit page:

<html>
<body>

<h1>DATABASE SERVER</h1>

<p>User Data Stored Here</p>

<p>Username: admin</p>
<p>Password: secret123</p>

</body>
</html>

🔥 PHASE 5 — CONNECT WEB → DATABASE

Now simulate backend call:

On Server2 (Web server)

Update HTML:

<html>
<body>

<h1>Company Portal</h1>

<a href="http://192.168.50.10">Access Database</a>

</body>
</html>

Test flow:

From PC:

http://200.1.1.2
→ click link
→ should open 192.168.50.10

🚨 NOW APPLY FIREWALL RESTRICTION (IMPORTANT)

We now enforce production behavior:

Requirement:

❌ Users cannot access DB directly
✅ Only Web Server can access DB

Step — FIX FIREWALL (Router)

Run:

conf t

no ip access-list extended INTERNAL_USERS

ip access-list extended INTERNAL_USERS

remark Allow users → web server only
permit tcp 192.168.0.0 0.0.255.255 host 200.1.1.2 eq 80

remark Block users → database
deny tcp 192.168.0.0 0.0.255.255 host 192.168.50.10 eq 80

permit ip any any

interface g0/0.10
ip access-group INTERNAL_USERS in

interface g0/0.20
ip access-group INTERNAL_USERS in

interface g0/0.30
ip access-group INTERNAL_USERS in

end

Now test:

From PC:

http://192.168.50.10

❌ SHOULD FAIL

From Web Server:

Server2 → Browser
http://192.168.50.10

✔️ SHOULD WORK

🔥 PHASE 6 — ADD PROXY (VERY IMPORTANT)

Now we simulate proxy:

👉 Proxy = control user internet access

Step — Make Server2 act as Proxy

In Packet Tracer (simplified):

Use HTTP filtering idea:

Update firewall:

conf t

ip access-list extended PROXY_CONTROL

remark Allow only web server access
permit tcp 192.168.0.0 0.0.255.255 host 200.1.1.2 eq 80

remark Block all other internet
deny ip 192.168.0.0 0.0.255.255 any

permit ip any any

interface g0/0.10
ip access-group PROXY_CONTROL in

interface g0/0.20
ip access-group PROXY_CONTROL in

interface g0/0.30
ip access-group PROXY_CONTROL in

end

Result:

Action	Result
PC → Web Server	✅
PC → Internet	❌
PC → DB	❌
Web → DB	✅

🔥 FINAL ARCHITECTURE (PRODUCTION STYLE)

[ USERS VLAN 10/20/30 ]
        ↓
     (Firewall)
        ↓
   [ DMZ - Web Server ]
        ↓
     (Firewall)
        ↓
 [ Private DB VLAN50 ]

🔥 PHASE 7 — SRE TROUBLESHOOTING SCENARIOS

Scenario 1 — Website not opening

Check:

ping 200.1.1.2

If fails:

show ip interface brief
show vlan brief

Scenario 2 — Page loads but DB not working

Check from Server2:

ping 192.168.50.10

If fails:

show access-lists

Scenario 3 — User cannot access web

Check:

show access-lists
show run | include access-group

Scenario 4 — DNS issue

Check:

ping 192.168.50.10

Then:

Server0 → DNS → ON

🔥 FINAL RESULT

You built:

✔️ VLAN segmentation
✔️ Router-on-a-stick
✔️ DMZ architecture
✔️ Firewall (ACL)
✔️ Proxy control
✔️ Web application
✔️ Database separation
✔️ SRE troubleshooting scenarios