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Silver

Seeking clarification on STP and the blocking port state please

Hi All

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,

Michael

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: Seeking clarification on STP and the blocking port state ple

Michael

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.

Jon

4 REPLIES
Hall of Fame Super Blue

Re: Seeking clarification on STP and the blocking port state ple

Michael

When a switch first becomes active it will start sending BPDU's on all it's port every 2 seconds. It will continue to send BPDU's on a particular port until/unless it hears of a better BPDU on that port. If it receives a better BPDU then it ceases to send it's own BPDUs.

So there is not wait period as such. A new switch will assume it is the root bridge until it receives a BPDU telling it isn't. If no BPDU is better then it will be the root bridge for that segment.

HTH

Jon

Silver

Re: Seeking clarification on STP and the blocking port state ple

Hi Jon

Thank you for the swift response and the information. This is what I originally understood from my NA studies and my initial NP studies on STP.

On a switch, it is my understanding that all ports are by default open (not administratively shout down), is this correct? So when I power on a switch, after POST and initialisation all ports that are not connected to end stations should transition to the blocking state and as you stated, it was my understanding that the switch would start to advertise itself as the root for all configured VLAN's until it received a superior BPDU which advertised a another switch with a better BID as the root bridge.

However a little further on in the material I am using I read that in blocking state a switch can only RECEIVE BPDU's, so I googled for some further information and the first link presented was a document from Cisco (linked to in original post) which seemed to agree with this receive only scenario. This is what caused the confusion.

Again many thanks for the swift response and clarification.

Best Regards,

Michael

Hall of Fame Super Blue

Re: Seeking clarification on STP and the blocking port state ple

Michael

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.

Jon

Silver

Re: Seeking clarification on STP and the blocking port state ple

Hi Jon

Cheers, thanks a million, MUCH appreciated Another mystery solved and the penny has finally dropped :)

Also thanks for the congrats. I have just noticed the silver star alongside my name.

Best Regards,

Michael

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