Load Balancer

⚖️ What is Azure Load Balancer?

Azure Load Balancer is a Layer 4 (Transport Layer - TCP/UDP) load balancer that distributes incoming traffic across multiple Azure resources (like virtual machines) for high availability and reliability.

It works at TCP/UDP level, unlike Application Gateway which works at HTTP/HTTPS (Layer 7).

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📚 Table of Contents

1. Azure Load Balancer Overview

2. Types of Azure Load Balancer

3. Key Components

4. Real-World Use Cases

5. Load Balancer vs Application Gateway

6. Step-by-Step Setup (Portal & CLI)

7. Health Probes

8. High Availability Ports

9. Best Practices

10. Interview Questions

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1️⃣ Azure Load Balancer Overview

Feature	Description
Layer	L4 (TCP/UDP)
Protocol	TCP/UDP
Routing	Port-based
Modes	Public / Internal
HA	Spreads traffic among VMs in a backend pool
Cost	Free (Basic), Pay for data (Standard)

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2️⃣ Types of Load Balancer

Type	Description	Use Case
Public	Exposes frontend IP to the internet	Inbound access to VMs
Internal	Uses private IP inside VNet	Internal app tiers, private services
Basic SKU	Legacy, no zone redundancy, limited	Dev/test
Standard SKU	Secure, scalable, supports availability zones	Production-grade apps

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3️⃣ Key Components

Component	Description
Frontend IP	Public or Private IP exposed to client
Backend Pool	Set of VMs or NICs to receive traffic
Health Probe	Checks availability of backend instances
Load Balancing Rule	Defines mapping between frontend and backend (port/protocol)
Inbound NAT Rules	Allow direct access to backend VMs (e.g., RDP/SSH)

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4️⃣ Real-World Use Cases

Scenario	Description
Web App Behind LB	Distribute HTTP traffic across multiple web servers
Internal Microservices	Use Internal LB for backend services
Active/Passive Failover	Use health probe for detecting node health
NAT Access	Use inbound NAT for SSH/RDP to backend VMs

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5️⃣ Load Balancer vs Application Gateway

Feature	Load Balancer	App Gateway
Layer	4 (TCP/UDP)	7 (HTTP/HTTPS)
Protocol Aware	❌	✅
Path Routing	❌	✅
SSL Termination	❌	✅
WAF	❌	✅
Use Case	Any TCP/UDP app	Web apps (HTTP/S)

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6️⃣ Step-by-Step Setup

✅ Using Azure Portal (Public LB)

1. Go to Create → Load Balancer

2. Choose:

   • SKU: Standard

   • Type: Public

   • Frontend IP: Create new Public IP

3. Create Backend Pool

   • Add your VMs

4. Create Health Probe

   • Port: 80 (or any running service)

5. Create Load Balancing Rule

   • Frontend: Port 80

   • Backend: Port 80

   • Probe: Use created probe

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💻 Azure CLI Example

# 1. Create public IP
az network public-ip create \
  --resource-group myRG \
  --name myPublicIP \
  --sku Standard

# 2. Create Load Balancer
az network lb create \
  --resource-group myRG \
  --name myLoadBalancer \
  --public-ip-address myPublicIP \
  --frontend-ip-name myFrontend \
  --backend-pool-name myBackendPool \
  --sku Standard

# 3. Add VMs to backend pool
az network nic ip-config address-pool add \
  --address-pool myBackendPool \
  --ip-config-name ipconfig1 \
  --nic-name myNic1 \
  --resource-group myRG \
  --lb-name myLoadBalancer \
  --frontend-ip-name myFrontend

# 4. Create Health Probe
az network lb probe create \
  --resource-group myRG \
  --lb-name myLoadBalancer \
  --name myHealthProbe \
  --protocol tcp \
  --port 80

# 5. Create Load Balancing Rule
az network lb rule create \
  --resource-group myRG \
  --lb-name myLoadBalancer \
  --name httpRule \
  --protocol tcp \
  --frontend-port 80 \
  --backend-port 80 \
  --frontend-ip-name myFrontend \
  --backend-pool-name myBackendPool \
  --probe-name myHealthProbe

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7️⃣ Health Probes

Probe Type	Used For
TCP	Checks TCP connectivity
HTTP	Checks app health via HTTP 200 OK
HTTPS	Secure health probe

Health probe removes unhealthy VMs from the backend pool automatically.

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8️⃣ High Availability Ports

• HA Ports allow all TCP/UDP traffic from a single rule.

• Use with Internal Load Balancer for SQL AlwaysOn, NVA scenarios.

Example use:

az network lb rule create \
  --name allPortsRule \
  --load-distribution Default \
  --protocol All \
  --frontend-port 0 \
  --backend-port 0 \
  --enable-tcp-reset true

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9️⃣ Best Practices

Practice	Description
Use Standard SKU	More secure, zone redundant
Use availability sets or zones	For HA of backend pool VMs
Use custom health probes	Validate app-specific health (e.g., HTTP 200)
Avoid Basic SKU for production	No autoscaling, no security features
Use NAT rules for RDP/SSH	Access individual VMs securely
Monitor with Azure Monitor + Log Analytics	For traffic visibility

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