Hi
We have 4 ESXi 5.5 Hosts with shared storage and all ESXi hosts are behind a hardware firewall.
On all ESXi Hosts there was one vmnic connected to the prod-dmz on a switch.
I wanted to add another switch for redundancy, so I did.
I connected on the ESXi Hosts a vmnic, used earlier by another vSS, to the vSS attached to prod-dmz on the second switch, on the external firewall the two switches are on two nics on the firewall with link aggregation.
Everything actually went well - but some vms lost the network. Out of 46 Servers, 7 lost the network, even though everything was "green", I didn't see any differences to all other servers. Also the guest operating systems were CentOS5, Windows Server 2008 and 2012 - other servers with the same OS didn't have the problem. Also the affected vms were on two different ESXi hosts.
The affected machines were able to ping themselves and other vms in the same DMZ but not the GatewayIP of the firewall internal nic. The other vms were able to ping and access everything.
It drove me nuts.
Than after some network troubleshooting I realized on a windows machine the warning that the media was disconnected.
What helped was changing the network adapter of the affected vms to another Switch, saving, editing it back to the wanted switch.
Some Linux machines needed a restart of the network, others didn't.
So just in case somebody changes nics between switches and has the same problem, this might help.
(btw if you cannot delete an old vSS after having moved all vms to another switch, remove the snapshots of the vms - if not possible rename the old switch first, afterwards you can delete it. The affected vms weren't the ones with snapshots in the above mentioned network problem).
cheers