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STP Blocking vs Forwarding

Lets say we have 3 switches all connected in a triangle form. SW1 is the root switch which connects to SW2 and SW3, and SW3 and SW2 also have a connection. What makes the decision as to whether SW2 or SW3 decides to block, Its connection between SW2 and SW3? Assuming that all links have a cost of 19.

I am not sure but I believe if the costs are the same to root, that it would then look at priority and if priority were the same it would look at mac and pick the higher of the 2 mac addresses between SW2 and SW3 (and assuming SW3 had the higher MAC) SW3 would block its connection to SW2. Can anyone verify I am on the right path here? Thanks!

1 ACCEPTED SOLUTION

Accepted Solutions

Re: STP Blocking vs Forwarding

Simple! You just have to remember what is in a BPDU. In descending precedence order you have: root ID, root path cost, sender bridge ID and sender port ID.

SW2 and SW3 send to each other BPDUs that have the same root ID and the same root path cost (assuming all links have a cost of 19 right?). Then, the tie-breaker between the two BPDUs is the sender bridge ID. As a bridge ID includes a mac-address, SW2 and SW3's IDs are different, and you are sure that one will beat the other.

BTW, a bridge ID = bridge priority + bridge mac address. So if you don't tune anything, the bridge with the worse (ie highest) mac address between SW2 and SW3 will block. You can however tune their bridge priority (you could also change the cost on their root port) if you want to influence the choice.

Regards,

Francois

2 REPLIES
Cisco Employee

Re: STP Blocking vs Forwarding

In this scenario, the link between SW2 and SW3 would be blocking. Depending on which is SW2 or SW3 in your diagram, one will have a higher Cost to the root and will block.

Re: STP Blocking vs Forwarding

Simple! You just have to remember what is in a BPDU. In descending precedence order you have: root ID, root path cost, sender bridge ID and sender port ID.

SW2 and SW3 send to each other BPDUs that have the same root ID and the same root path cost (assuming all links have a cost of 19 right?). Then, the tie-breaker between the two BPDUs is the sender bridge ID. As a bridge ID includes a mac-address, SW2 and SW3's IDs are different, and you are sure that one will beat the other.

BTW, a bridge ID = bridge priority + bridge mac address. So if you don't tune anything, the bridge with the worse (ie highest) mac address between SW2 and SW3 will block. You can however tune their bridge priority (you could also change the cost on their root port) if you want to influence the choice.

Regards,

Francois

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