STP – Loop itself with BPDUfilter

We investigate a special topology condition, where a switch is connected itself on ports Faa0/1 and Fa0/2, and one of the port has BPDUFilter feature applied (i.e. simulating STP is switched off). Both ports are in the same vlan (vlan 1 here). The question is: Is there a topology loop?

Initial state

We check first interfaces status, where we should see that both ports are connected and operational

Switch#sh int status

Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        connected    1          a-full  a-100 10/100BaseTX
Fa0/2                        connected    1          a-full  a-100 10/100BaseTX

similarly for observing the spanning tree state of VLAN1:

Switch#sh span vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     001d.e5bc.0580
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     001d.e5bc.0580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Back BLK 19        128.2    P2p

Where we may see, that in usual condition STP has blocked the Fa0/2 port. The decision is based on worse BPDU, which is send out of the Fa 0/2 port (sender PID is the key).

Now we will apply BPDUFilter on fa 0/2

Switch(config-if)#span bpdufilter enable

and reset the port (apply shutdown and no shutdown because port remains on previous STP state). After that we may observe, that the

port Fa0/2 moves through STP states to the forwarding state

Switch(config-if)#do sh span

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     001d.e5bc.0580
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     001d.e5bc.0580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

So the result is that the bridged loop has arised and has influence on a switch and network performance.

Note: the situation is the same if we apply BPDUfilter on the fa0/1 port.