Virtual machines behind a load balancer receive uneven traffic distribution

Summary

This article explains why virtual machines (VMs) in the backend pool of an Azure Load Balancer might not receive traffic evenly. It provides guidance on how to identify configuration problems that can affect load-balancing behavior and steps to verify that traffic is distributed as expected across the backend VMs.

Azure Load Balancer distributes traffic based on connections. Check traffic distribution per connection and not per packet. Verify distribution by using the Flow Distribution tab in your preconfigured Load Balancer Insights dashboard.

Azure Load Balancer doesn't support true round robin load balancing but supports a hash based distribution mode.

If you suspect backend pool members aren't receiving traffic evenly, it could be due to the following causes.

Cause 1: You configured session persistence

Using source persistence distribution mode can cause an uneven distribution of traffic. If this distribution isn't desired, update session persistence to be None so traffic is distributed across all healthy instances in the backend pool. For more information, see distribution modes for Azure Load Balancer.

Cause 2: You configured a proxy

Clients that run behind proxies might appear as one unique client application from the load balancer's point of view.