Cross-zone load balancing in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Cross-Zone Load Balancing Behavior

What is the main effect of enabling cross-zone load balancing on an AWS Application Load Balancer?

A It disables health checks for targets in other Availability Zones.

B It restricts traffic to targets only within the same Availability Zone as the load balancer node.

C It evenly distributes incoming traffic across all registered targets in all enabled Availability Zones.

D It routes all traffic to the target group with the lowest latency regardless of zone.

💡 Hint

Think about how traffic is balanced when targets are unevenly distributed across zones.

Enabling cross-zone load balancing allows the load balancer to distribute traffic evenly across all registered targets in all zones, preventing uneven load when targets are not evenly spread.

❓ Architecture

intermediate

Designing for High Availability with Cross-Zone Load Balancing

You have an AWS Network Load Balancer with targets unevenly distributed: 3 targets in Zone A and 1 target in Zone B. Cross-zone load balancing is disabled. What is the expected traffic distribution?

A Traffic is split evenly between Zone A and Zone B, but Zone B targets get more traffic per instance.

B Traffic is split evenly between all targets regardless of zone.

C Traffic is routed only to Zone B targets.

D All traffic is routed only to Zone A targets.

💡 Hint

Consider how load balancers distribute traffic when cross-zone load balancing is off and targets are uneven.

With cross-zone load balancing off, the load balancer distributes traffic evenly across Availability Zones, not targets. Zone B has fewer targets, so each target there receives more traffic.

❓ Configuration

advanced

Enabling Cross-Zone Load Balancing via AWS CLI

Which AWS CLI command correctly enables cross-zone load balancing for a Network Load Balancer named my-nlb?

AWS

aws elbv2 modify-load-balancer-attributes --load-balancer-arn <load-balancer-arn> --attributes Key=load_balancing.cross_zone.enabled,Value=true

A aws elbv2 modify-load-balancer-attributes --load-balancer-arn <load-balancer-arn> --attributes Key=load_balancing.cross_zone.enabled,Value=true

B aws elbv2 set-load-balancer-attributes --name my-nlb --cross-zone-load-balancing enabled

C aws elbv2 update-load-balancer --name my-nlb --cross-zone true

D aws elbv2 enable-cross-zone --load-balancer-name my-nlb

💡 Hint

Look for the correct AWS CLI command and attribute syntax for modifying load balancer attributes.

The correct command uses modify-load-balancer-attributes with the attribute key load_balancing.cross_zone.enabled set to true.

❓ security

advanced

Security Implications of Cross-Zone Load Balancing

What is a potential security consideration when enabling cross-zone load balancing on an AWS load balancer?

A It disables encryption between the load balancer and targets.

B It may increase inter-AZ data transfer costs and expose traffic to more network paths, increasing attack surface.

C It automatically opens all inbound ports on targets for cross-zone traffic.

D It disables AWS WAF protections on the load balancer.

💡 Hint

Think about how traffic flows between zones and what that means for security and costs.

Cross-zone load balancing causes traffic to flow between Availability Zones, which can increase data transfer costs and expose traffic to more network paths, potentially increasing the attack surface.

❓ service_behavior

expert

Impact of Cross-Zone Load Balancing on Auto Scaling Behavior

You have an Auto Scaling group behind an Application Load Balancer with cross-zone load balancing enabled. If one Availability Zone loses all its instances, what happens to the traffic distribution?

A Traffic to the affected zone is dropped until instances are replaced manually.

B The load balancer disables cross-zone load balancing automatically.

C The load balancer continues to send traffic evenly to all zones, causing errors in the empty zone.

D The load balancer automatically routes all traffic to instances in the remaining zones without manual intervention.

💡 Hint

Consider how the load balancer handles target health and zone availability.

With cross-zone load balancing enabled, the load balancer detects unhealthy or missing targets in a zone and routes traffic to healthy targets in other zones automatically.

Practice

1. What is the main benefit of enabling cross-zone load balancing in AWS Elastic Load Balancer?

easy

A. It increases the maximum number of connections per target.

B. It automatically scales the number of Availability Zones.

C. It evenly distributes incoming traffic across all registered targets in all enabled Availability Zones.

D. It encrypts all traffic between the load balancer and targets.

Solution

1. Step 1: Understand cross-zone load balancing purpose

   Cross-zone load balancing spreads traffic evenly across targets in all zones, preventing uneven load.

2. Step 2: Compare options to this purpose

   Only It evenly distributes incoming traffic across all registered targets in all enabled Availability Zones. describes this even distribution. Other options describe unrelated features.

3. Final Answer:

   It evenly distributes incoming traffic across all registered targets in all enabled Availability Zones. -> Option C

4. Quick Check:

   Cross-zone load balancing = even traffic spread [OK]

Hint: Cross-zone means traffic spreads evenly across zones [OK]

Common Mistakes:

• Confusing cross-zone with encryption or scaling
• Thinking it increases max connections per target
• Assuming it changes number of zones automatically

2. Which of the following is the correct way to enable cross-zone load balancing in an AWS Application Load Balancer using Terraform?

easy

A. enable_cross_zone_load_balancing = false

B. cross_zone_load_balancing = true

C. cross_zone_enabled = true

D. enable_cross_zone_load_balancing = true

Solution

1. Step 1: Recall Terraform attribute for cross-zone load balancing

   The correct attribute is enable_cross_zone_load_balancing and it must be set to true to enable.

2. Step 2: Match options to correct syntax

   Only enable_cross_zone_load_balancing = true uses the exact attribute name and sets it to true.

3. Final Answer:

   enable_cross_zone_load_balancing = true -> Option D

4. Quick Check:

   Terraform attribute = enable_cross_zone_load_balancing true [OK]

Hint: Look for exact attribute name with true value [OK]

Common Mistakes:

• Using incorrect attribute names like cross_zone_load_balancing
• Setting the value to false instead of true
• Confusing with other unrelated settings

3. Given the following AWS CLI command to enable cross-zone load balancing on a Classic Load Balancer:

aws elb modify-load-balancer-attributes --load-balancer-name my-load-balancer --load-balancer-attributes '{"CrossZoneLoadBalancing":{"Enabled":true}}'

What will be the effect of running this command?

medium

A. Cross-zone load balancing will be disabled for the load balancer.

B. Cross-zone load balancing will be enabled, distributing traffic evenly across zones.

C. The load balancer will be deleted.

D. The command will fail due to syntax error.

Solution

1. Step 1: Analyze the AWS CLI command structure

   The command modifies load balancer attributes, specifically enabling CrossZoneLoadBalancing by setting Enabled to true.

2. Step 2: Understand the effect of enabling CrossZoneLoadBalancing

   Enabling it causes traffic to be evenly distributed across all registered targets in all zones.

3. Final Answer:

   Cross-zone load balancing will be enabled, distributing traffic evenly across zones. -> Option B

4. Quick Check:

   Enable CrossZoneLoadBalancing true = even traffic spread [OK]

Hint: Enabled true means cross-zone load balancing is active [OK]

Common Mistakes:

• Thinking the command disables the feature
• Assuming the load balancer is deleted
• Believing the command syntax is incorrect

4. You configured cross-zone load balancing on your Network Load Balancer but notice traffic is still uneven across zones. What is the most likely cause?

medium

A. You forgot to register targets in all Availability Zones.

B. Cross-zone load balancing is not supported on Network Load Balancers.

C. Cross-zone load balancing requires manual DNS updates.

D. You must enable cross-zone load balancing on each target instance.

Solution

1. Step 1: Recall cross-zone load balancing support on Network Load Balancers

   Network Load Balancers support cross-zone load balancing, but it must be enabled and targets must be registered in all zones.

2. Step 2: Identify why traffic is uneven

   If targets are missing in some zones, traffic cannot be balanced evenly despite cross-zone load balancing being enabled.

3. Final Answer:

   You forgot to register targets in all Availability Zones. -> Option A

4. Quick Check:

   Targets missing in zones = uneven traffic [OK]

Hint: Ensure targets exist in all zones for balancing [OK]

Common Mistakes:

• Thinking cross-zone load balancing is unsupported on NLB
• Believing DNS updates are needed
• Assuming enabling on targets is required

5. You have an Application Load Balancer spanning three Availability Zones with cross-zone load balancing enabled. One zone has only one healthy target, while the others have three each. How does cross-zone load balancing affect traffic distribution?

hard

A. Traffic is evenly distributed across all targets regardless of zone count.

B. Traffic is evenly distributed across zones, so the zone with one target gets one-third of traffic.

C. Traffic is sent only to zones with the most targets.

D. Traffic is distributed randomly without considering zones or targets.

Solution

1. Step 1: Understand cross-zone load balancing behavior

   With cross-zone load balancing enabled, each load balancer node distributes traffic evenly across all healthy registered targets, regardless of their Availability Zone.

2. Step 2: Apply to given scenario

   Total healthy targets: 1 + 3 + 3 = 7. Each target receives approximately equal traffic (1/7), so the zone with one target gets about 1/7 of total traffic.

3. Final Answer:

   Traffic is evenly distributed across all targets regardless of zone count. -> Option A

4. Quick Check:

   Cross-zone load balancing = even across all targets [OK]

Hint: Cross-zone balances evenly across all targets, ignoring zone boundaries [OK]

Common Mistakes:

• Confusing with default zone-isolated balancing (even per zone)
• Thinking traffic prefers zones with more targets
• Believing distribution ignores target count