I have an odd one.
I have an ESXi 5.1 host with a newly upgraded ESXi 5.5 vCenter server. After the upgrade, everything worked fine. It was the smoothest upgrade I had done with vCenter, or even most any software in general. I was happy about this. It worked great for three weeks after the upgrade.
On Saturday, I brought down the host, which includes most of our test lab infrastructure, including our DNS server, Lab AD DC, and our vCenter server, to add memory. I upgrade it from 40GB to 64GB, which is the max for the machine. When I powered it up, it had a problem where it was disconnected from the vCenter server. I originally thought it would be because the vCenter server was busy loading the database. So, I thought nothing of it. However, after everything loaded, it still keeps disconnecting the host. I tell it to connect, it does, updates the stats, and then immediately drops again.
All I could find on an internet search were mentions of a host disconnecting in 30-60 seconds because of network interference, but this can't be with this machine. The Management interface, vCenter server, AD DC, DNS server, and backup servers are all on the same virtual network switch connected to two on-board NICs, with the rest of our infrastructure VMs on two addon NICs. So, if there is network interference, it would have to be at the virtual switch level. All network traffic between the two in totally internal. I can manage the host from the software client without issue, and I can manage the vCenter server from the web client or the software client without issue. Pings between them go through in "<1ms". There doesn't seem to be any network interference at all.
I cannot figure out why this is happening. I can still manage the host, but with it disconnected from the vCenter server, our backups won't happen. So, I need to get this fixed soon.
Any ideas?