Yes this port is an alternate port. This means you will be having inbound BPDU's. If you want to check where the BPDU's are coming from you can do a packet capture on the port filtering for: ether host 01:80:c2:00:00:00 and let it run for 30 seconds so you can gather all BPDU's. The bridge ID of the incoming BPDU should give you a clue from what the device the frame is coming from. ... View more