Virtual machine lost network connectivity due to duplicate IP addresses

Products

• VMware vSphere ESXi

Issue/Introduction

When a duplicate IP exists on a network, one or more of the following behaviors may be observed: 

• A virtual machine is unable to ping its gateway IP.
• The command ipconfig on a Windows VM shows the word "(duplicate)" next to an IP.
• The commands ipconfig or ifconfig shows the same IP configured on more than one device.
• A pop-up message appears on a Windows client OS alerting the user to a duplicate IP.
• On Windows operating systems, an alert is recorded in Windows Event Viewer with event ID 4199.
• When doing a ping or other connectivity test to the VM, we observe that the source MAC address on the reply packets does not match the MAC address of a network interface belonging to the target VM. Replies are seen from a different MAC address - the device sending replies also has the same IP and its MAC address may match another VM in the environment.
• If a firewall is in use, for instance a Palo Alto, the firewall may log "Received conflicting ARP" errors on it's interfaces.

To identify the MAC address and/or location of the device with the duplicate, you can:

• Review VMs and ports in vSphere; the MAC address can be seen when VM settings are edited.
• Check the host(s) with the net-stats -l command.
• Examine the switch CAM/MAC table.
• Review the event log in the operating system, Windows will report the MAC address of the conflicting device.
• Perform a packet capture - filtering for EtherType 0x0806 will show only ARP data.

Note: MAC addresses that begin with 00:0c:29 or 00:50:56 generally indicate a VMware OUI.

Cause

The same IP address has been assigned to two virtual machines or devices in the same network.

Resolution

• To avoid packet loss, one of the devices with the duplicate IP should be reconfigured to a different, unused IP.
• You can change the IP address settings for a VM inside of the operating system.
• The virtual NIC adapter can also be disconnected in vCenter, disabled in the OS or removed from the VM if it is not needed.

Additional Information

During power outages, check with your network teams for configuration changes that weren't saved properly.

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