Could not reach isolation address: x.x.x.x

I was applyling patches to one of my esx clusters (vSphere 4.0) but after taking one of my ESX hosts out of Maintenance Mode, I realised that HA was not enabled. When I looked into the tasks and events for this ESX host, I saw the error “Could not reach isolation address: x.x.x.x”.

The IP address mentioned in this error was the IP of a physical switch that the network  guys had  removed a few weeks ago. This IP is the means by which the ESX hosts check each other for availability and is configured by the setting “das.isolationaddress”. If your network team has locked down PING to the Default Gateway, you will need to use this.

This is where you modify the das.isolationaddress setting:

1.  Right click on the Cluster.
2. Select “Edit Settings”.
3. Select “VMware HA”.
4. Select “Advanced Options…” on the right.
5. For “das.isolationaddress”, Enter a valid IP address for your physical switch.
6. Ok.

After modifying this setting, I right clicked on each ESX host and ran through “Reconfigure for VMWare HA”.

After doing this, HA was back…