Understanding Spanning-Tree Protocol

Answered Question
Jun 16th, 2010

Hello


A port in the blocking state performs as follows:


  • Discards frames received from the attached segment.



  • Discards frames switched from another port for forwarding.



  • Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)



  • Receives BPDUs and directs them to the system module.



  • Does not transmit BPDUs received from the system module.



  • Receives and responds to network management messages.


When it says "Does not transmit BPDU's" does that mean just that specific switch is not transmitting, because there must be a switch sending out some sort of BPDU in order for the STP to be working right? I assume they are describing a switch alone by itself does not transmit anything. However if there was other switches already running STP successfully then it would transmit it's BPDU's to the new switch turned on?


Please advise.


Thanks.


Joe



Reference the following document for the source information: http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Correct Answer by Jon Marshall about 6 years 8 months ago

When it says "Does not transmit BPDU's" does that mean just that specific switch is not transmitting, because there must be a switch sending out some sort of BPDU in order for the STP to be working right? I assume they are describing a switch alone by itself does not transmit anything. However if there was other switches already running STP successfully then it would transmit it's BPDU's to the new switch turned on?


Please advise.


Thanks.


Joe



Reference the following document for the source information: http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm


Joe


It is per port not per switch. So a port that is blocked receives but does not transmit BPDUs but there may well be other ports on the switch that are forwarding BPDUs.


Jon

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Correct Answer
Jon Marshall Wed, 06/16/2010 - 11:25

When it says "Does not transmit BPDU's" does that mean just that specific switch is not transmitting, because there must be a switch sending out some sort of BPDU in order for the STP to be working right? I assume they are describing a switch alone by itself does not transmit anything. However if there was other switches already running STP successfully then it would transmit it's BPDU's to the new switch turned on?


Please advise.


Thanks.


Joe



Reference the following document for the source information: http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm


Joe


It is per port not per switch. So a port that is blocked receives but does not transmit BPDUs but there may well be other ports on the switch that are forwarding BPDUs.


Jon

Mohamed Sobair Wed, 06/16/2010 - 13:35

Adding to jon reply,


A port in STP blocking state relies on continous recieved BPDU, if the port for any reason failed to recieve those BPDUs , the port will immediately transit to STP forwarding state, hence causing spanning tree bridging loop.


A port in STP blocking state doesnt forward any BPDUs.



HTH

Mohamed

nelson.garcia Wed, 06/16/2010 - 13:47

Mohamed, I'm certain that a port in blocking state does not immediately transition to forwarding state when it stops receiving BPDUs. It waits for timers to expire, those of which I can't recall off the top of my head (max age, 2x forward delay?), which is about 50 seconds.


Good ways to get around the STP convergence time would be to switch to RSTP, or use Uplink fast on the access layer which has <5 second convergence time and brings up the blocked port immediately.

Mohamed Sobair Wed, 06/16/2010 - 14:00

Yes nelson you are correct, I should have mentioned that.


yes for STP 802.1d, it waits for max age timer 20 sec + 2 x 15 sec forward delay timer which is 50 sec.



HTH

Mohamed

joealbergo Thu, 06/17/2010 - 23:05

Thank you everyone for your responses -


I think all your information is extremely helpful and have given everyone the five stars!



Thanks again... you all are great ---

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