Task Statement 2.1: Implement routing and connectivity between on-premises networks and the AWS Cloud.

📘AWS Certified Advanced Networking – Specialty

📌 What is Load Balancing?

Load balancing is the process of distributing incoming network traffic across multiple servers or resources to:

Improve performance
Increase availability
Prevent overload on a single server
Ensure fault tolerance

🔹 Why it matters in AWS exam

In AWS networking, load balancing is critical because:

Applications run on multiple EC2 instances
Traffic must be evenly distributed
High availability across multiple Availability Zones is required

🔷 Types of Load Balancing by OSI Layers

Load balancing works at different layers of the OSI model. For the exam, you must clearly understand:

Layer 3 (Network Layer)
Layer 4 (Transport Layer)
Layer 7 (Application Layer)

🔶 1. Layer 3 Load Balancing (Network Layer)

📌 Definition

Layer 3 load balancing makes decisions based on:

IP addresses (source/destination)

📌 How it works

Uses routing techniques
Traffic is forwarded based on destination IP
Does NOT inspect ports or application data

📌 Characteristics

Very fast (minimal processing)
Limited intelligence
No awareness of applications

📌 Example in IT environment

Routers distributing traffic across multiple network paths
AWS VPC route tables influencing traffic direction

📌 AWS relevance

Route tables
Transit Gateway routing decisions
Equal-cost multi-path (ECMP)

📌 Key exam point

👉 Layer 3 = IP-based routing decisions only

🔶 2. Layer 4 Load Balancing (Transport Layer)

📌 Definition

Layer 4 load balancing distributes traffic based on:

IP address
TCP/UDP port numbers

📌 How it works

Looks at connection-level data
Does NOT inspect application content
Works with TCP and UDP

📌 Characteristics

High performance and low latency
Supports millions of requests
Limited intelligence (no content awareness)

📌 Example in IT environment

Distributing incoming TCP connections to multiple backend servers
Handling database or application traffic without inspecting content

📌 AWS Service

👉 Network Load Balancer (NLB)

🔹 Features of NLB

Ultra-high performance
Static IP addresses
Preserves client IP
Handles TCP, UDP, TLS

📌 Key exam point

👉 Layer 4 = connection-based load balancing (IP + port)

🔶 3. Layer 7 Load Balancing (Application Layer)

📌 Definition

Layer 7 load balancing makes decisions based on:

HTTP/HTTPS headers
URLs
Cookies
Application data

📌 How it works

Inspects full request content
Routes traffic based on rules

📌 Characteristics

Intelligent routing
Content-aware decisions
Slightly higher latency than L4

📌 Example in IT environment

Routing /api requests to API servers
Routing /images requests to image servers
Directing traffic based on hostname

📌 AWS Service

👉 Application Load Balancer (ALB)

🔹 Features of ALB

Path-based routing
Host-based routing
Works with HTTP/HTTPS
Supports WebSockets

📌 Key exam point

👉 Layer 7 = content-based routing

🔷 Comparison: Layer 3 vs Layer 4 vs Layer 7

Feature	Layer 3	Layer 4	Layer 7
Decision based on	IP	IP + Port	Content (HTTP/HTTPS)
Speed	Fastest	Very fast	Slower (more processing)
Intelligence	Low	Medium	High
AWS usage	Routing tables	NLB	ALB
Content awareness	❌ No	❌ No	✅ Yes

🔷 Reverse Proxy

📌 Definition

A reverse proxy is a server that:

Receives client requests
Forwards them to backend servers
Returns the response to the client

👉 The client does NOT communicate directly with backend servers.

📌 How it works

Client sends request to reverse proxy
Reverse proxy decides which backend server to use
Backend processes request
Reverse proxy sends response back to client

📌 Functions of Reverse Proxy

Load balancing
SSL/TLS termination
Security (hide backend servers)
Caching
Request filtering

📌 AWS Example

🔹 Application Load Balancer (ALB)

Acts as a Layer 7 reverse proxy

🔹 Network Load Balancer (NLB)

Not a full reverse proxy (passes traffic directly)

📌 Key exam point

👉 Reverse proxy = client talks to proxy, not directly to backend

🔷 AWS Load Balancer Types (Important for Exam)

AWS provides three main load balancers:

🔶 1. Application Load Balancer (ALB)

Layer 7
HTTP/HTTPS only
Content-based routing
Best for web applications

🔶 2. Network Load Balancer (NLB)

Layer 4
TCP/UDP/TLS
Ultra-high performance
Best for low-latency applications

🔶 3. Gateway Load Balancer (GWLB)

Layer 3 + Layer 4 (special case)

📌 Purpose

Used for security appliances
Works with:
- Firewalls
- Intrusion detection systems

📌 Key Feature

Uses GENEVE protocol
Enables transparent traffic inspection

🔷 Advanced Concepts (Exam Important)

🔹 1. Cross-Zone Load Balancing

Distributes traffic evenly across all AZs
Improves availability

🔹 2. Health Checks

Load balancer checks backend server health
Removes unhealthy instances automatically

🔹 3. Sticky Sessions (Session Affinity)

Sends same user to same backend server
Useful for stateful applications

🔹 4. SSL/TLS Termination

Load balancer handles encryption/decryption
Reduces backend server load

🔹 5. Connection Draining (Deregistration Delay)

Allows in-progress requests to complete before removing instance

🔷 When to Use What (Exam Scenarios)

📌 Use Layer 3

Routing between networks
No application awareness needed

📌 Use Layer 4 (NLB)

High performance required
TCP/UDP traffic
Low latency applications

📌 Use Layer 7 (ALB)

Web applications
Need routing based on URL/headers
Microservices architectures

📌 Use Reverse Proxy

Hide backend infrastructure
Add security and control
Centralized request handling

🔷 Exam Tips (Very Important)

✅ Know differences:

L3 vs L4 vs L7
NLB vs ALB vs GWLB

✅ Remember:

ALB = Layer 7 + Reverse Proxy
NLB = Layer 4 + High performance
GWLB = Security appliances

✅ Key keywords in questions:

“URL-based routing” → ALB
“Low latency / TCP” → NLB
“Firewall inspection” → GWLB

🔷 Final Summary

Layer 3 → IP-based routing
Layer 4 → Connection-based (IP + port)
Layer 7 → Content-based routing
Reverse proxy → intermediary between client and servers
AWS uses:
- ALB (Layer 7)
- NLB (Layer 4)
- GWLB (Layer 3/4 for security)