Cisco switch not communicating with VLAN [RESOLVED]

[nLinked]

We have a Cisco 6513 connected to a HP 5412 switch. Port GigabitEthernet1/25 on the Cisco has the following settings:

 

no ip address

switchport

switchport access vlan 100

switchport mode access

spanning-tree portfast

spanning-tree bpduguard enable

 

A device plugged into that port was able to communicate to devices on vlan 100 just fine for the last few months. But just after rebooting the Cisco switch recently, any device connected to that port no-longer can communicate. We have made no changes to its config, and the running config before the reboot was saved. What could have made it lose connectivity with vlan 100 just by rebooting? Thanks.

Edited April 10, 2014 by nLinked

[SYNACK]

If it is receiving a bounce back of the SPF broadcast it will block the port, perhaps there is something a miss in your HP core config or a dodgey device plugged in somewhere to VLAN 100 causing it to think there is a loopback on this port and to needs to be shutdown. The switches are not connected in any other way that may be allowing the SPF broadcast to bounce back?

 

You can turn on SPF debugging on the cisco to have a look or just set debug all however this will probably cripple the switch performance wise and you will need to set the debug output to whatever method you are using to connect.

[nLinked]

dodgey device plugged in somewhere to VLAN 100 causing it to think there is a loopback

 

That did it! I remember now we were testing another device via a small unmanaged switch, but this was plugged into the HP not the Cisco. Anyhow, unplugging that switch resolved communication from the Cisco switch. It must have been going through this little unmanaged switch, getting lost. Thanks!