Blocking state in STP?

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Jun 21st, 2010
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Hello, a little confused. In STP, can a port in the Blocking state SEND and Receive BPDUs or it can only receive BPDUs? My understanding is that it should also send BPDUs, because if sudenly there is a change in topology that causes that specific port to send a better BPDU to the segment it should be able to become a designated port. The book says that it receives but it doesn't say it sends BPDUs, that's why I ask. For the other states it does says it sends and receives BPDUs.


Thanks.

Correct Answer by Federico Coto F... about 6 years 11 months ago

Good questions!


In scenario 1, if there's a new link between switch B and the root, then Switch B will have an STP convergence (because there has been a topology change), and therefore there will be an STP recalculation and most likelly the port that was blocking to switch A will now transition to another state where it can send BPDUs and announce switch A of the better path.


In scenario 2, I think you're correct.


Federico.

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Federico Coto F... Mon, 06/21/2010 - 14:34
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Hi,


In a blocking state it only receives BPDUs (it does not send).

This is to make the port aware when there's a topology change and that it needs to change state and perhaps become active.


In all other STP states, the port sends and receives BPDUs.


Federico.

fjcardenas-1 Mon, 06/21/2010 - 15:58
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Lets see. Lets say 3 switches are connected to the same segment, at the begining all of them will anounce their root path cost to the segment and they will all agree that one of them (let's say SW-A) wins and it will have the designated port. SW-B and SW-C will have their ports in Blocking state. The designated port (SW-A) will be able to send and receive BPDUs, but Blocking ports in SW-B and SW-C will ONLY receive BPDUs.


Thinking in this scenario #1........If a new link is activated between Root and SW-B, and therefore SW-B now can announce a better BPDU to the segment... doesn't it announce the better BPDU to the segment? (considering a port in blocking state only receives BPDUs).... or how does it work in this case?


Thinking of another scenario #2...... if suddenly something changes in the topology and now SW-A sends BPDUs with a higher root path cost to the segment...... in this case the blocking port in SW-B will receive this higher BPDU..... will SW-B compare this received BPDU to its own root path cost.... compares them, and decide to move the port to the forwarding state?


I hope I am clear with my questions.


Thanks.

Correct Answer
Federico Coto F... Mon, 06/21/2010 - 16:10
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Good questions!


In scenario 1, if there's a new link between switch B and the root, then Switch B will have an STP convergence (because there has been a topology change), and therefore there will be an STP recalculation and most likelly the port that was blocking to switch A will now transition to another state where it can send BPDUs and announce switch A of the better path.


In scenario 2, I think you're correct.


Federico.

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