Knowing which devices are sending BPDU packets on the network when troubleshooting spanning tree issues can be helpful. This allows you to identify the root bridge and non-switch devices that may be impacting the spanning tree.
On Cisco switches, we can use the following command to get detailed spanning tree information for a specific interface:
show spanning-tree interface {interface} detail
For example:
PIT-LKL-CORES1# show spanning-tree interface ethernet 1/5 detail Port 4507 (Ethernet1/5) of VLAN2001 is root forwarding Port path cost 1, Port priority 128, Port Identifier 128.4507 Designated root has priority 32771, address 00AB.CDEF.1234 Designated bridge has priority 32771, address 0012.3456.7890 Designated port id is 128.4507, designated path cost 1 Timers: message age 0, forward delay 0, hold 0 Number of transitions to forwarding state: 3 Link type is point-to-point by default BPDU: sent 12345, received 56789 Port 4507 (Ethernet1/5) of VLAN2002 is root forwarding Port path cost 1, Port priority 128, Port Identifier 128.4507 Designated root has priority 32772, address 00AB.CDEF.1234 Designated bridge has priority 32772, address 0012.3456.7890 Designated port id is 128.4507, designated path cost 1 Timers: message age 0, forward delay 0, hold 0 Number of transitions to forwarding state: 3 Link type is point-to-point by default BPDU: sent 23456, received 67890
Looking at the BPDU section, we can see this port is sending BPDUs and receiving them. This indicates another device on the other end of this link participating in spanning tree.
By checking this detail on switch interfaces, we can identify all the devices sending BPDUs and their role in the spanning tree topology. This is useful when troubleshooting unexpected topology changes or hunting down a rogue device impacting the network.
Knowing exactly who sends BPDUs provides valuable insight into what devices influence the spanning tree state. This simple command can save hours of headaches tracking down the source of spanning tree issues.
Let me know if you want any changes to this example output or blog post text. I’m happy to update it as needed.