Well, this is something really embarrassing.
I have upgraded our vSphere to version 5 about 3 months ago and it is only NOW I found out that I haven't upgraded Distributed vSwitch. To be honest, I even didn't know I had to do it manually, I was convinced it would be upgraded during the ESXi hosts' upgrade and very probably I overlooked this fact in the official documentation.
It doesn't impact your infrastructure at all, however your Distributed vSwitch will lack cool new features like NetFlow, Port Mirroring and User-Defined network resource pools.
So, if you missed this part too just go to dvSwitch, where you will notice Yellow warning mark next to Upgrade link. The upgrade process will check if your hosts are compatible with dvSwitch 5.0. After another couple of clicks you will have it upgraded.
This just proves I am getting too rusty on vSphere due to absence of practice with it. I think I need to change my job :)
