Expected behavior for Load-Based Teaming (LBT) port reassignment in vSphere

Products

• VMware vCenter Server

Issue/Introduction

This article clarifies the expected behavior for in-flight TCP sessions and UDP streams during a Load-Based Teaming (LBT) port reassignment convergence window in a vSphere environment.

Environment

• VMware vSphere Distributed Switch (vDS)
• Load-Based Teaming (LBT) policy enabled

Cause

LBT reassigns a virtual port (dvPort) to a different physical uplink (vmnic) when mean Tx or Rx utilization on the current uplink exceeds 75% over a 30-second sampling window. This action triggers a network convergence event to update the upstream physical switch's MAC address table.

Resolution

1. Understand the convergence: LBT port reassignment is faster than a classic vMotion because the virtual machine is not paused or migrated, it is a path failover where traffic is rerouted to a different physical adapter.
2. TCP versus UDP handling: There is no difference in how the host or physical switch treats TCP or UDP protocols during this event. The networking stack treats all traffic as standard Layer 2 frames. The ESXi host does not perform session-aware processing or special handling for TCP or UDP traffic during this reassignment.
3. Physical switch requirements: Because this is a network convergence event, the upstream physical switch must update its MAC address table to associate the virtual machine's traffic with the new physical uplink.
4. Recommendations for latency-sensitive workloads:
   • If workloads are strictly latency-sensitive, LBT might introduce unnecessary network jitter. Consider evaluating a different load balancing policy to avoid these convergence events.
   • If you find that convergence time is higher than expected, investigate physical switch convergence settings. For information on minimizing failback delays, see Lost pings and delays in the physical switch during NIC teaming failback (310812).

Additional Information

For more details on network teaming and failover policies, see the vSphere Networking documentation.
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