STP Convergence

[daudparvez]

I have read in CISCO CCNP book that when a  switch first turns on, it put all it's ports into Blocking state for 20 sec in which it will only hear the BPDUs from other switches. But if all the switches are turned on simultaneously, how will they get any BPDUs? Also, will the user ports will be blocked or not? and how does a switch knows that a port is connected to a switch or to a user?

Also, if a topology change occurs,on a switch will only the CAM table entries will be flushed or all the ports of a switch will be Bloced? and a rejudgement of RP and DP will be done?

[Peter Paluch]

Hello Daud,

The book is bluntly wrong on the subject.

The default state of a newly enabled port in STP is Listening. So after you power on switches, all ports will jump into Listening state, sending and receiving BPDUs. Of course, after exchanging at most a few BPDUs, it will be clear which ports are redundant, and they will fall into Blocking state, while the remaining ports will proceed through Listening, Learning into Forwarding.

If a topology change occurs, the primary result is the CAM content flushing. The RP and DP do not change or re-elect basing on the sole event of topology change, rather, their role changes basing on the contents of received BPDUs.

Feel welcome to ask further!

Best regards,

Peter