I have made a diagram at this pastebin - basically i have a production network with a pair of 6840 as my core (VSS), it has port channels to various systems. I do not use any routing protocols, VTP is set transparant across the board, and rapid spanning tree is enabled, no port security enabled, no NAC of any kind - very default in most settings really. A closet switch 3850 stack is connected to it with a trunk port channel. From this i have a 3560 on my desk, connected with a trunk port. This all works great.
I was then going to configure a 2960S for some testing at my desk, off of the 3560 permanently on my desk I have an access port on vlan 50.
With the 2960 i configured all ports as mode access, access on vlan 1, disabled spanning tree, stopped cdp and lldp, vtp transp, other than that it had no configuration.
When i plugged it to my desk 3560, all ports on the 350 immediately go "orange", and I found out it also happened to the up stream, as if the port channel between the 6840 and the 3850 was blocked. I gather it must be spanning tree or something kicking in, but I really don't get why. Because it is a production network i was not able to keep playing with it, as soon as I unplugged it was back to normal within 30 seconds.
Doing show logging on all of them did not really reveal anything to me.
as a side note - it took the switch to a lab that has a pair of 3560's - plugging it in there had no issues at all, worked as I expected there.
is there some hidden magic I am not getting here related SPT - or what is the proper way to connect such as switch in a "dumb access port" way?
Edit: Added config files for the 6840 core, the closet "poe stack" 3850, and my desk 3560. I do not have the 2960/config available, i had to configure and ship, but that config was very vanilla, and the switch port was exactly as described in diagram. Also added an updated diagram that has interface info so it can be found in the config.
Files: https://transfer.sh/dSSDr/core6840.txt https://transfer.sh/PHXLM/desk3560.log https://transfer.sh/5auAo/diagram.txt https://transfer.sh/iSiVF/poe3850.txt
I dont know if this helps to try figure out the "downage" cause? was hoping someone just had a hunch...
