Troubleshooting network connectivity issues using Address Resolution Protocol (ARP)

Products

VMware vSphere ESXi

Issue/Introduction

This article provides information to help you troubleshoot ESX network connectivity issues using ARP.

Symptoms:

A virtual machine cannot connect to the network.
Virtual machine tasks fail because of network disconnection.
ARP table does not show proper Media Access Control (MAC) addresses.

Environment

VMware ESX Server 3.0.x
VMware ESX 4.1.x
VMware ESX Server 3.5.x
VMware ESX 4.0.x

Resolution

The ARP table is a dynamic system table that is built when virtual machines communicate over TCP/IP with other nodes. The ARP table automatically maps the IP address of a virtual machine to the MAC address (the network adapter's unique embedded address). After a period of time, if virtual machine A no longer communicates with virtual machine B, the system ages out the entry in the ARP table.

Virtual machines use ARP table entries to address packets to send to other nodes.

The ARP cache is a list of recently resolved IP addresses to MAC address mappings. If an entry in the ARP cache is incorrect, the IP datagram may be sent to the wrong computer. You must clear the ARP cache.

To clear the ARP cache:

To list the ARP table, run the command:

arp -a
To populate the ARP table on a certain interface, run this command from the service console to display all mappings currently in the ARP cache:

arping -I vswifX IP_address

Where IP_address is the IP address of the virtual machine.
To clear all entries, run the command:

arp -d IP_address

Where IP_address is an internet address stored in the ARP cache.

Run this command for each entry in the ARP cache until all entries have been deleted. When the ARP cache is empty, you see the message:

No ARP Entries Found
To populate the ARP table, ping a broadcast IP address to get a reply back from all hosts listening on the same subnet.

For example:

ping 10.10.10.255
Use the tcpdump command to monitor TCP/IP traffic. For more information, see Verifying gateway IP connection using the tcpdump command (1008017).

Note: In ESXi tcpdump is changed to tcpdump-uw

For information specific to ESXi 5.5, see Clearing the ARP cache table in ESXi 5.5 (2049667).

Additional Information

For related troubleshooting information, see Troubleshooting virtual machine TCP/IP connection issues (1007842).

Notes:

This article applies only to ESX. It does not apply to ESXi. The only method of clearing the ARP cache on an ESXi 4.1 - 5.1 host is to reboot.
In ESXi 4.1, you can list the ARP table using this command:

esxcli network neighbor list
In ESXi 5, you can list the ARP table using this command:

esxcli network ip neighbor list
In ESXi 5.5, the ARP table now can be cleared by typing the command:

esxcli network ip neighbor remove

For more information on:

ARP commands for ESXi, see Displaying the ARP and Neighbor Discovery cache for VMkernel network interfaces (2000130).
ESXi 5.5 Network commands, see the VMware vSphere 5.5 Documentation Center .

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