Network failure on ESXi hosts after a PNIC failover and the Team-PNIC status on esxtop is in Fallback state

Products

• VMware vSphere ESXi

Issue/Introduction

• Virtual Machines (VMs) may lose all network access immediately after being moved to a specific ESXi host.
• While the host appears healthy, any VM residing on it cannot send or receive traffic, resulting in a total loss of service for those specific workloads.

Environment

VMware vSphere ESXi

Cause

Team-PNIC for the workloads were marked as "Fallback"

[root@esxmgmt-01a:~] netdbg vswitch instance list
  Client                         PortID          DVPortID                             MAC                  Uplink           VID    VNI
 TestVM.eth0              67108877        49                                   00:50:56:xx:xx:xx    Fallback     0-4094   

The "Fallback" status indicates a Link Aggregation (LAG/LACP) negotiation failure. This happens when the physical switch is expecting an active LACP bond, but the newly reinstalled host is treating the uplinks as standalone connections. Because the "handshake" fails, the uplinks are marked as inactive for data transfer.

Resolution

To resolve the "Fallback" state, re-align the host uplinks with the existing Link Aggregation Group (LAG):

1. Navigate to the Distributed vSwitch in management console.

2. Manage the host's networking and ensure the physical adapters are assigned to the correct LAG ports.

3. Monitor the status until the "Fallback" warning clears and the uplinks show as Active.