Intermittent VM network packet loss or ping drops due to duplicate MAC addresses

Products

• VMware vSphere ESXi

Issue/Introduction

• Intermittent ping loss (packet drops) for specific virtual machines (VMs).
• Issue persists even when migrating VMs between different ESXi hosts.
• Packet captures (pktcap-uw) may show ARP requests egressing the host, but ARP replies are missing or inconsistent.
• Physical switch logs may report "MAC flapping" or "MAC address moved" between ports.

Environment

• VMware Cloud Foundation (VCF) 4.x / 5.x
• VMware vSphere ESXi 7.x / 8.x
• VxRail

Cause

This issue occurs when two or more virtual machines are assigned the same MAC address. This causes the physical switch to frequently update its MAC address table (MAC flapping), leading to intermittent packet delivery failures as the switch attempts to route traffic to the incorrect port.

Resolution

To resolve this issue, identify the duplicate MAC addresses and reconfigure the impacted VMs with unique identifiers.

1. Identify duplicate MAC addresses within the vCenter inventory
2. Power off the impacted VM(s).
3. Right-click the VM and select Edit Settings.
4. Expand the Network Adapter section.
5. Change the MAC Address from "Manual" to Automatic (or manually assign a known unique address).
6. Power on the VM and verify connectivity.
7. If the issue was reported to a physical switch team, confirm that MAC flapping logs have ceased for those specific ports.