802.1d - Can Blocking Ports Hold BPDUs?

[Juan Urti]

Blocking Ports don´t send BPDUs, but they can receive them from designated ports.

Blocking ports, can it hold BPDUs?

I think that Blocking Ports only receive, analyse and then discard the BPDU.

But, what happen if the blocking port receive a better o worse BPDU? in this case, must be the bpdu stored?

Thanks from Argentina.

Juan

[Juan Urti]

The final answer is difficult.

If the blocking port receive a worse bpdu, it wait for the better bpdu in his root port.

If the BP receive a better BPDU, he inmediatly has become the root port.

Never hold the bpdu, he only see and discard it.

[Rolf Fischer]

¡Hola Juan!

Every port has to store the information of its best BPDU for MaxAge timer (20 sec by default). This information is always compared to the BPDUs  received on this port.

If no BPDU is received, the best BPDU is the one which the brigde has generated itself for that port, its "own" BPDU so to speak. This is the one it sends out every Hello interval.

A blocked port will remain in blocking state as long as inferior BPDUs (compared to the switch generated BPDU) are received for the port. If no BPDUs are received, after MaxAge, the port will transition to listening state.

When a new (different) BPDU is received it depends on the topology. If the port will become now Root Port or Designated Port, the STP topology is changed and the port will transition to listening, learning, forwarding. Otherwise it will remain in blocking state.

HTH

Rolf