Found an interesting scenario where I was asked to connect a regular 3550 switch to a Nexus 2k switch. I thought to myself sure, why not. All I have to do is make sure I forward the right VLANs, hardcode the trunk and I’m set right?
Well if your still reading this you know that isnt the case! By design, Cisco Nexus 2ks have BPDUGuard turned on all its interfaces and as far as I know you CANT turn them off. 2ks are meant to connect to hosts only, not other switches, hence the logs you see when you try to connect a switch
2K# 2014 Jan 20 18:27:46 NY-N5K-D06-1 %STP-2-BLOCK_BPDUGUARD: Received BPDU on port Ethernet131/1/46 with BPDU Guard enabled. Disabling port.
2014 Jan 20 18:27:47 NY-N5K-D06-1 last message repeated 2 times
2014 Jan 20 18:27:47 NY-N5K-D06-1 %ETHPORT-2-IF_DOWN_ERROR_DISABLED: Interface Ethernet131/1/46 is down (Error disabled. Reason:BPDUGuard)
One fix is to connect the switch to a N5k instead, but of course you’d need the right Twinax cable and they arent exactly as cheap as the regular ethernet cables. Or you could BUY another 2k, but the simple fix is to disable BPDUs on the 3550 switch. Managment as yet to say if they want to do this, a few forums I found mention its “OK” to do, but the jury is still out on that one.