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Here's the situation: There is a bridge that has the lowest ID within a loop. This bridge has two ports, both with equal cost to the root, how does STP decide which port is forwarding, and which port is blocking? Does it find the port with the highest ID and block on that port?

If anyone needs more information, I'd be happy to provide it. Thanks in advance for your help.


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1 Answer 1

The root port is always the link directly connected to the root bridge, or the shortest path to the root bridge. If more than one link connects to the root bridge, then a port cost is determined by checking the bandwidth of each link. The lowest-cost port becomes the root port. If multiple links have the same cost, the bridge with the lower advertising bridge ID is used. Since multiple links can be from the same device, the lowest port number will be used.

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For some additional reading around port priority in tie situations, i found the following off of google. – Steve Feb 3 '13 at 22:02

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