BPDU Filtering question

[Sheraz_35]

Hi, I am a bit confused regarding BPDU filtering, is the below correct?

If a port with BPDU filtering and Portfast enabled receives a BPDU then the port status will change and portfast will be disabled and so will BPDU filtering then the port will participate in STP like a normal port?

If BPDU filtering is enabled on a port without portfast then when the port receives BPDU's it will ignore them and BPDU filtering will stay enabled and this has the potential to cause a loop? 

Thanks

[Peter Paluch]

Sheraz,

No, the rules you have posted are not correct. The behavior of BPDU Filter depends on how/where it is configured - either in the global configuration mode, or directly on a port.

• If configured globally using spanning-tree portfast bpdufilter default command, BPDU Filter applies only to ports that have PortFast enabled, and the operation of the BPDU Filter on such ports is as follows:
  
  After the port comes up, it will send a limited number of BPDUs (10-11) to potentially negotiate the STP operation with an attached switch (there should be none but life is not fair ;) ). After that, the port will stop sending BPDUs. If at any time, either during the initial period of sending BPDUs or afterwards, a BPDU is received, both BPDU Filter and PortFast will be disabled on this particular port, and the port will start acting as a normal non-edge port whose STP role and state will be determined by usual STP rules.
• If configured on a port using the spanning-tree bpdufilter enable command, the presence of PortFast on this port is irrelevant to BPDU Filter operation. This port will not send any BPDUs and will drop all received BPDUs without processing them. Even if a BPDU arrives, it will be dropped before any other systems will see it, meaning that neither STP nor PortFast will know about this received (and dropped) BPDU, and none of them will react.

BPDU Filter has the greatest potential of creating switching loops when configured directly on a port because that port effectively stops participating in STP and it can never be blocked by it.

Best regards,
Peter

[Sheraz_35]

Thank you for the very good explanation Peter!