Azure Load Balancer (Layer 4) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Azure Load Balancer (Layer 4)?

Azure Load Balancer (Layer 4) is a service that distributes incoming network traffic across multiple virtual machines or services based on transport layer protocols like TCP and UDP, helping to improve availability and scalability.

beginner

Which OSI layer does Azure Load Balancer operate on?

Azure Load Balancer operates on Layer 4, the Transport Layer, which means it routes traffic based on IP address and TCP/UDP ports without inspecting the content of the packets.

intermediate

What are the two types of Azure Load Balancer?

Azure Load Balancer has two types: Basic and Standard. Basic is for small-scale or dev/test workloads, while Standard supports higher scale, availability zones, and more features.

intermediate

How does Azure Load Balancer handle health probes?

Azure Load Balancer uses health probes to check the status of backend instances. If an instance fails the probe, it stops sending traffic to that instance until it becomes healthy again.

intermediate

What is the difference between Public and Internal Azure Load Balancer?

Public Load Balancer routes traffic from the internet to your VMs, while Internal Load Balancer routes traffic within a virtual network or from on-premises networks, keeping traffic private.

Which protocol does Azure Load Balancer use to distribute traffic?

A HTTP and HTTPS

B DNS and DHCP

C FTP and SMTP

D TCP and UDP

Azure Load Balancer works at Layer 4 and distributes traffic based on TCP and UDP protocols.

What happens if a backend VM fails the health probe in Azure Load Balancer?

A Traffic is still sent to the VM

B Traffic is stopped to that VM until it recovers

C The VM is deleted automatically

D The Load Balancer restarts

Azure Load Balancer stops sending traffic to unhealthy VMs until they pass health probes again.

Which Azure Load Balancer type supports availability zones?

A Basic

B Classic

C Standard

D None

Standard Load Balancer supports availability zones for higher availability.

What is the main difference between Public and Internal Azure Load Balancer?

A Public routes internet traffic; Internal routes traffic within a virtual network

B Public is free; Internal is paid

C Public supports TCP; Internal supports UDP

D Public is Layer 7; Internal is Layer 4

Public Load Balancer handles internet traffic, while Internal Load Balancer routes traffic privately inside a virtual network.

Azure Load Balancer operates at which OSI model layer?

A Layer 4 - Transport

B Layer 3 - Network

C Layer 2 - Data Link

D Layer 7 - Application

Azure Load Balancer works at Layer 4, the Transport Layer.

Explain how Azure Load Balancer improves application availability and scalability.

Think about how spreading traffic helps keep apps running smoothly.

Describe the difference between Basic and Standard Azure Load Balancer.

Consider scale, features, and reliability differences.

Practice

1. What is the primary function of the Azure Load Balancer (Layer 4)?

easy

A. Encrypt data at rest in Azure storage

B. Inspect and modify HTTP headers for incoming requests

C. Store and manage user session data

D. Distribute incoming TCP/UDP traffic evenly across multiple servers

Solution

1. Step 1: Understand Layer 4 Load Balancing

   Azure Load Balancer works at the transport layer (Layer 4) to distribute TCP/UDP traffic.

2. Step 2: Identify its main role

   It balances traffic across multiple servers to improve availability and scalability without inspecting message content.

3. Final Answer:

   Distribute incoming TCP/UDP traffic evenly across multiple servers -> Option D

4. Quick Check:

   Layer 4 Load Balancer = TCP/UDP traffic distribution [OK]

Hint: Layer 4 means TCP/UDP traffic balancing only [OK]

Common Mistakes:

• Confusing Layer 4 with Layer 7 load balancer features
• Thinking it inspects HTTP headers
• Assuming it manages session data

2. Which of the following is the correct way to configure a backend pool in Azure Load Balancer (Layer 4)?

easy

A. Configure SSL certificates in the backend pool

B. Assign virtual machines or VM scale sets to the backend pool

C. Add HTTP routes to the backend pool

D. Set up DNS names in the backend pool

Solution

1. Step 1: Understand backend pool composition

   Backend pools in Azure Load Balancer contain virtual machines or VM scale sets to receive traffic.

2. Step 2: Eliminate incorrect options

   HTTP routes, SSL certificates, and DNS names are not configured in backend pools for Layer 4 load balancer.

3. Final Answer:

   Assign virtual machines or VM scale sets to the backend pool -> Option B

4. Quick Check:

   Backend pool = VMs or VM scale sets [OK]

Hint: Backend pool holds VMs or VM scale sets only [OK]

Common Mistakes:

• Trying to add HTTP routes to backend pool
• Confusing SSL setup with load balancer config
• Adding DNS names instead of VMs

3. Given an Azure Load Balancer configured with 3 healthy backend VMs, what happens when one VM becomes unhealthy?

medium

A. Traffic is routed only to the 2 healthy VMs

B. Traffic is evenly distributed to all 3 VMs regardless

C. Load balancer stops routing traffic until all VMs are healthy

D. Traffic is routed only to the unhealthy VM

Solution

1. Step 1: Understand health probe role

   Azure Load Balancer uses health probes to detect unhealthy VMs and stops sending traffic to them.

2. Step 2: Apply health probe behavior

   When one VM is unhealthy, traffic is routed only to the remaining healthy VMs to maintain availability.

3. Final Answer:

   Traffic is routed only to the 2 healthy VMs -> Option A

4. Quick Check:

   Unhealthy VM excluded from traffic [OK]

Hint: Unhealthy VMs do not get traffic [OK]

Common Mistakes:

• Assuming traffic still goes to unhealthy VMs
• Thinking load balancer stops all traffic
• Believing unhealthy VMs get all traffic

4. You configured an Azure Load Balancer but clients report intermittent connection failures. What is a likely cause?

medium

A. Backend VMs have too much CPU capacity

B. Load balancer is inspecting HTTP headers incorrectly

C. Health probes are misconfigured, marking healthy VMs as unhealthy

D. DNS names are missing in the backend pool

Solution

1. Step 1: Analyze symptoms

   Intermittent connection failures often relate to backend availability issues.

2. Step 2: Identify misconfiguration impact

   If health probes are misconfigured, healthy VMs may be marked unhealthy, reducing available servers and causing failures.

3. Final Answer:

   Health probes are misconfigured, marking healthy VMs as unhealthy -> Option C

4. Quick Check:

   Misconfigured health probes cause connection failures [OK]

Hint: Check health probe settings first for connection issues [OK]

Common Mistakes:

• Blaming backend VM CPU capacity without evidence
• Thinking Layer 4 load balancer inspects HTTP headers
• Assuming DNS names belong in backend pool

5. You want to design a highly available web service using Azure Load Balancer (Layer 4). Which combination ensures scalability and fault tolerance?

hard

A. Use a backend pool with multiple VM scale sets and configure health probes

B. Use a single VM with static IP and no health probes

C. Configure DNS round-robin without load balancer

D. Use Azure Load Balancer with only one backend VM and no health probes

Solution

1. Step 1: Identify scalability needs

   Multiple VM scale sets allow automatic scaling of backend servers to handle load.

2. Step 2: Ensure fault tolerance

   Health probes detect unhealthy instances and route traffic only to healthy ones, improving availability.

3. Step 3: Evaluate other options

   Single VM or no health probes reduce fault tolerance; DNS round-robin lacks health checks and load balancing features.

4. Final Answer:

   Use a backend pool with multiple VM scale sets and configure health probes -> Option A

5. Quick Check:

   Scale sets + health probes = scalable, fault tolerant [OK]

Hint: Combine scale sets with health probes for best availability [OK]

Common Mistakes:

• Using single VM reduces fault tolerance
• Ignoring health probes causes traffic to unhealthy VMs
• Relying on DNS round-robin lacks health checks