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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.

-Josh

jerney
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2 Answers2

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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.

Lucas Kauffman
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  • For some additional reading around port priority in tie situations, i found the following off of google. http://astorinonetworks.com/2011/06/30/spanning-tree-port-priority/ – Steve Feb 03 '13 at 22:02
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I ended up on this post from Google, and Lucas Kauffman's answer is wrong. "The root port is always the link directly connected to the root bridge".

That's not true. Look at the following topology : STP

Multilayer Switch 1 is the Root bridge. You can see that Multilayer Switch 3 is directly connected to it, but its port is blocked nonetheless. That's because Multilayer Switch 3 is connected with a FastEthernet link to Multilayer Switch 1, whereas all other links are Gigabit Ethernet links.

So, the shortest (least hop) path is NOT selected for the Root port, even if directly connected. You should always calculate the cost to the Root Bridge. In this case, 2x Gigabit links give a cost of 4+4 = 8. The FastEthernet link has a cost of 19. Hence, the Root Port is the link towards Multilayer Switch 2.

Julien
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