Ran into this issue late last week, where a customer with a multi-site design had ESX hosts in Site A show up as disconnected in vCenter. The vCenter server runs in Site B-- and they just turned up a new circuit between the sites.

VMware has a pretty good KB article on how to troubleshoot issues with ESX(i) hosts going disconnected in vCenter (<cite>kb.vmware.com/kb/1003409). </cite>I've been down that path before, and had a good idea of what to look for, but nothing seemed to be "wrong". I was able to ping between host and vCenter (host to vCenter, or vCenter to host), resolve names, verified correct management IPs, restarted managed services, etc.

So after spending about 45 minutes troubleshooting and scratching my head, I ran a few protocol analyzers to see what was actually happening on the wire-- and pretty quickly saw what the issue was.

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The ESX host "heartbeats" are all apparently sent at 1500 MTU-- and I'd see heartbeats sent from the ESX host, but never arrive on the other end. Anything over about 1460 bytes was getting fragmented/never arriving. After calling the provider they verified that they weren't able to pass anything over 1460 bytes, reconfigured some stuff on their end, and viola, problem solved!