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stp scenario

Answered Question
Apr 6th, 2012
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Hi everybody.




Please consider the following scenario ( My apology for messy diagram):


The network is convergred and a switch located inside (not shown) switched network is elected as root bridge.









1) Let say for for some reasons Sw1 stops receiving bpdu on its f1/1 port.  Let say 30 seconds have passed, How will sw1 react?  Will  sw1 transition its f1/3 blocked port to forwarding state or will it put f1/3 through learning and listening states first?



2)  Please consider the above diagram.  Network has converged with root switche inside the switched network. Let say someone connects a new switch with a lower priority.


As a result sw1 receives a superior bpdu on its f1/1. Sw3 also receives a superior BPDU on its f1/2 port.

Sw3 forwards the bpdu out of f1/3; sw1 also forwardsthe bpdu out of its port f1/3. Sw1 concludes the bpdu it has transmitted out of f1/3 is superior and decides f1/3 as designated port.  The question is while all this happening,will sw1 continue to forward user frames?



thanks and have a great weekend

Correct Answer by maxime.gerges about 5 years 4 months ago

I think that SW1 blocked port would move trough listenning state and exchange BPDU with SW3 to decide which port will be the new designated port as a new root bridge has been heard. The election will be :


  • lowest root bridge ID (the same in this case)
  • lowest root path cost.
  • lowest sender bridge ID.
  • lowest sender port ID.
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maxime.gerges Sat, 04/07/2012 - 02:27
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For the 1st question I would say that SW1 has lost connectivity with the root bridge out its root port. After the Max Age timer expires (20s per default), SW1 start to consider its f1/3 port to be the root port but SW1 still needs to move from listenning to learning to forwarding in order to avoid bridging loop to occur so the network will reconverge in ~50seconds.


For the 2nd question, its a bit tricky so i would say that SW1 and SW3 will have to move their port trough listenning and forwarding state as a new root bridge is heard. Anyone can confirm ?

sarahr202 Sat, 04/07/2012 - 07:24
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Thanks Maxime.


  Please consider it as a part of 2nd question. 

  Having received superior bpdu ( new switch witl lower priority) on its f1/1, how will sw1 deal with its blocked f1/3 port since  it is a blocked port, no bpdu are sent out? 


thanks and have agreat weekend.

Correct Answer
maxime.gerges Sat, 04/07/2012 - 09:07
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I think that SW1 blocked port would move trough listenning state and exchange BPDU with SW3 to decide which port will be the new designated port as a new root bridge has been heard. The election will be :


  • lowest root bridge ID (the same in this case)
  • lowest root path cost.
  • lowest sender bridge ID.
  • lowest sender port ID.
sarahr202 Sat, 04/07/2012 - 14:06
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I found the answer for the second question. Question 1 has already been answered,


2)  Please consider the above diagram.  Network has converged with  root switch inside the switched network. Let say someone connects a new  switch with a lower priority.


As a result sw1 receives a superior bpdu on its f1/1. Sw3 also receives a superior BPDU on its f1/2 port.

Sw3  forwards the bpdu out of f1/3; sw1 also forwardsthe bpdu out of its  port f1/3. Sw1 concludes the bpdu it has transmitted out of f1/3 is  superior and decides f1/3 as designated port.  The question is while all  this happening,will sw1 continue to forward user frames?



If both switches sw1 and sw2 have the same root path cost to newly introduced root switch, then following will occur:



Sw1 receives a superior bpdu on its f1/1. Sw1 's f1/3 is still blocked. The dcision whether to unblock f1/3  and transition it through listening,learning and forwarding state will be dependent upon  who sends the superior bpdu on f1/3 segment.

Since sw1's f1/3 is still blocked and therefore no bpdu can be sent out . However sw1 can still decides whether to unblock f1/3 or not.  Sw1 continues to receive bpdu from designated bridge sw3 on its f1/3 blocked port. Since both sw1 and sw3   root path cost  is same,  therefore sender bridge id is ised to break the tie.  So when sw1 receives the bpdu from sw3, ( assuming sw3 bridge id is higher than sw1's bridge id), it concludes it  could transmit superior bpdu out of f1/3(blocked port). Sw1 then transition the blocked port through llistening ,learning and forwarding state.




if both switches sw1 and sw2 have different root path cost to newly introduced root switch. Following will occur


Let say sw1's root path cost is higher than sw3's root path cost.


Sw1 receives the superior bpdu from  new root switch on its root port f1/1

Sw1 also receives the bpdu on its f1/3 blocked port from sw3. This bpdu is superior b/c root path cost  is lower than that of sw1. Sw1 therefore leave f1/3 in blocked states.



.


The switch will not forward user traffic while its all ports or one port are going through transitionary states ( listening,learning).  The switch will only forwards user frame if its port are in stable states  which are blocked or forwarding. If any port is going through listening or learning states, switch will not forward user frames.



Thanks and have a great weekend







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