VMs lose network connectivity with VMware Tools and HCX MON enabled

Products

• VMware HCX

Issue/Introduction

• Virtual machines migrated via HCX Bulk Migration experience a total loss of network connectivity approximately 30–40 seconds after booting in the target environment. This issue occurs when Mobility Optimized Networking (MON) is enabled and the target NSX segment utilizes a custom Virtual Distributed Router (VDR) MAC address.

  Symptoms:

  • VM network connectivity is stable for the first 30–40 seconds of uptime.
  • Network drops completely once the VMware Tools service initializes and the guest OS drivers fully load.
  • Stopping or disabling the VMware Tools service restores connectivity.
  • The issue specifically impacts environments where the target gateway uses a non-default NSX VDR MAC.

Environment

• VMware HCX
• VMware NSX (Target environment with custom VDR MAC)
• VMware Tools active in Guest OS

Cause

• HCX MON uses VMware Tools to inject /32 host routes into the guest OS to optimize traffic paths. These routes are designed to point to the standard VMware VDR MAC. When a custom VDR MAC is configured on the target segment, the injected routes point to an incorrect Layer 2 destination. This results in an invalid ARP state, blocking outbound traffic.

  Note on Supportability:

  • Custom VDR MACs on the source environment are supported across all HCX versions.
  • Custom VDR MACs on the target environment are only supported starting with HCX 9.1.

Resolution

To restore network connectivity for affected VMs, use the following steps to disable MON:

1. Log in to the HCX Manager UI.
2. Navigate to the Network Extension tab.
3. Locate the extended network segment and find the affected virtual machine(s).
4. Manually disable Mobility Optimized Networking (MON) for the specific VM.
5. If MON is not required for the segment, it can be disabled at the segment level to prevent this issue for future migrations.

Additional Information

HCX Bulk Migration operations and best practices