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How to clear the ARP cache in ESXi prior to vSphere 5.5

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Yesterday, I wrote an article about a new ESXCLI command that will be available as part of the soon to be released vSphere 5.5 release which allows a user to clear the ARP cache on an ESXi 5.5 host. It was my understanding that with past releases of ESXi, that it is not possible to clear the ARP cache. While working on that article, I came across an internal thread and learned that it was possible to clear ARP entries prior to ESXi 5.5, however the method is not as ideal as using the new ESXCLI command.

Disclaimer: This is not officially supported by VMware, please use at your own risk

In ESXi 5.1, you can list the current ARP entries for an ESXi host by running the following ESXCLI command:

esxcli network ip neighbor list

To clear a particular ARP entry, you will need to use the unsupported vsish interface. To delete a specific ARP entry, you will need to run the following command:

vsish -e set /net/tcpip/v4/neighbor del [IP-ADDRESS]

Here is a screenshot of listing the current ARP entries using ESXCLI and then deleting one of entries using vsish:

As mentioned in my previous article, I am glad we have made this into an official command in ESXCLI 5.5, but if you are in a crunch you can still clear an ARP entry if you are not running ESXi 5.5 using vsish.

How to clear the ARP cache on ESXi using new ESXCLI 5.5

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The ability to clear the ARP cache/table for an ESXi host is feature that has been requested from customers from time to time and usually for diagnosing network related issues. In previous releases, this was not something you could easily do and it was nice to see this feature get implemented in a new ESXCLI command that will be available as part of the vSphere 5.5 release.

The ESXCLI "network ip neighbor" namespace has been enhanced to include a new option:

You can list ARP table entries using the "list" operation and you can now clear a specific ARP entry using the "remove" operation.

To clear an ARP entry, you will need to specify the IP Address you wish to clear, the IP protocol whether it is IPv4 or IPv6 and you also have the option to specify the particular VMkernel interface. If you do not specify the particular VMKernel interface, then the entry will be removed for all interfaces.

Here is a screenshot of listing of the current ARP entries and then removing an entry for VMkernel interface 1:

This will definitely be a useful ESXCLI command to be aware of the next time you need to troubleshoot a network issue!

Note: I mentioned earlier that clearing the ARP cache was not something that you could do prior to vSphere 5.5. However, while working on this article, I actually learned it was possible to clear the ARP cache, but it was not very user friendly like the new ESXCLI 5.5 command. In a separate article, I will show you how can clear the ARP cache on earlier versions of ESXi.

If you wish to clear the ARP cache on an ESXi host prior to ESXi 5.5, please take a look at this article for more details.