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STP question

Would be a bit confusing, but probably draw a diagram and it will make sense.

If there are 3 switches, A,B&C. Assume Swtich A is the root bridge. The link between A&B is 10Mbps, A&C is 100Mbps, C&B is 100Mbps.

In this case, all the ports on A is set to "forwarding". The port on C which connects to A is set to "forwarding" since it is where it receives the low cost helo BPDU. The port on B connecting to C receives the lowest cost helo BPDU, it is set to "forwarding".

What happens to the Port on C, which connects to B.

My actual question is can 2 ports on a non-root switch be set to "forwarding".

Apologies if my question doesn't makes sense, I am still in learning state. :-(

Reply to
raj
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The basic STP algorithm is:

1) Elect the root switch - switch A in your example. 2) On non-root switches, a) elect the root port (lowest cost to the root switch) Switch A will select the A-C link since the cost is the 100Mbps (as compare to the C-B-A link (cost 100MB). For simplicity, let say the cost of the 100MBps = 1 and the 10MBps = 10, the cost for the A-C link is then 1 and the cost for the C-B-A link = 1 + 10 or 11.

Switch B will select the B-C-A since the cost is 2 (100Mbps +100Mbps) where as the B-A link is 10!

b) elect the designate port the local LAN segment (lowest cost wins). On the B-C link, C wins because C has lower cost.

So, in you example, the two ports on switch A will be "forwarding"; two ports on switch C will be "forwarding; the port from B to C will be "forwarding" while the port from B to A will be "blocking".

Doan

Reply to
Doan

Makes a lot of sense now. Thanks Doan.

Reply to
suresh zen

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