- Silver, 250 points or more
A little confused with STP and the Blocking port state.
In my CCNA studies I just took what I read as gospel and learned it. However, in my NP studies I question what I read and sometimes find out that things I took as gospel I don't fully understand and this is not good.
I have read in my NP study material that the only thing that a trunk port in blocking state can do is receive BPDU's from neighbors. This is also stated in this Cisco Spanning Tree document http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm which goes one step further and states that a port in blocking state "Does not transmit BPDU's received from the system module", which I take to mean it does not transmit BPDU's to its neighbors. This I find a little strange.
If I added a new switch into a network running STP, would this new switch not immediately start SENDING BPDU's announcing itself as the root bridge? as this was kind of alluded to earlier in the material.
Or is it the case that it must wait until it receives a BPDU telling it who the advertised root switch is. It would then check its own BID against that of the advertised root bridge and only if its own local BID was lower than the advertised root BID would it start sending BPDU's announcing itself as the root bridge? And if its own BID was higher than the advertised root bridge BID, it would learn from the received BPDU what state this port should transition to.
When I think about it a little more this "wait and see" option does make sense, however I would appreciate someone in the know setting me straight :)
Best Regards & TIA,
You have gone all silver :-), congratulations.
I think i see where the confusion is and i may have mislead you. I think the key bbit from the doc is
"After initialization, a BPDU is sent to each port in the switch. A switch initially assumes it is the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is really the root. If only one switch resides in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A switch always enters the blocking state following switch initialization."
So a single BPDU is transmitted when the port becomes active. If no BPDU's are heard on that port then the port transitions to learning. If a BPDU is received the port transitions to blocking and will not send any more BPDU's until it transitions from that state.
Apologies if i mislead you.