Azure load balancer - n-tier application

Below the best practice you may consider:

1. Use Zone-Redundant Frontend IPs

• Recommended: Use zone-redundant frontend IPs for your Standard Load Balancer.
• This ensures the LB is not tied to a single zone, and traffic can be routed to healthy instances across zones.
• This is critical for resiliency in case one AZ becomes unavailable.

2. Single vs Multiple Load Balancers

• Single Load Balancer is sufficient and recommended for most n-tier apps:
• Reduces complexity and cost
• Easier to manage and monitor
• Still supports multiple frontends and backend pools
• Multiple Load Balancers may be justified if:
• You need different SLAs or configurations per tier
• You want tier isolation for security or compliance
• You have different traffic patterns or protocols per tier

3. Backend Pool Configuration

• Ensure each backend pool includes VMs from multiple zones.
• Use availability zone-aware rules and health probes to detect failures and reroute traffic.

4. HA Ports and Floating IP

• Use HA Ports for backend rules if you're handling non-HTTP(S) traffic or need port agnostic load balancing.
• Enable Floating IP if you're using SQL Always On or failover clustering in the data tier.

5. NSG and UDR Considerations

• Ensure NSGs and User Defined Routes (UDRs) do not block health probe traffic.
• Allow traffic from Azure Load Balancer IP: 168.63.129.16 for health probes.

Azure Load Balancer Best Practices - Azure Load Balancer | Microsoft Learn

Azure Load Balancer and Availability Zones - Azure Load Balancer | Azure Docs