STP question

I'm running practice exercises as I prep for the ICND2 and I'm struggling a bit with understanding this particular one.

Here's the topology:

Here's the configuration information:

And here's the switch output:

The exercise asks for me to predict the port status and behavior. I thought I had good understanding of STP but evidently I'm missing something, based on the ports available I believe that F0/1 should be blocked because it has a root cost of 8 (cost of 4 to cross the link to SW1, cost of 4 to cross the link from SW1 to SW2) which is higher than the root cost of the G0/1 port which is 2, but according to the switch out F0/1 is a designated, forwarding port.

I tried rereading the chapter to see what I'm missing and couldn't find anything concrete, so I'm turning to here for a bit of clarity on why SW4's F0/1 port is a designated, forwarding port. Is it simply because there has to be a designated port on one side of the link and SW1's F0/1 link is blocked, causing SW4's F0/1 to become the designated port in spite of its root cost? Or is there something else I'm missing? Thank you in advance for your help.

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Comments

pevangel

On a given link between two switches, there is a designated bridge. The designated bridge will have its port as forwarding and the other switch will be blocking. For the link between switch 4 and switch 1, switch 4 is the designated bridge due to the lower path cost to the root bridge.

davenull

I updated the topology with the link costs so it's easier to see what's going on:

Switch 1 and switch 4 already have root ports out of Gi0/1's, so the link between them will have one port forwarding and one blocking. How to determine which switch wins:

1. The switch with the lowest cost to root wins. If the costs are equal:
2. The switch with the lowest bridge ID wins. If the IDs are equal (2 links to the same switch):
3. The port that received a lower port ID (port priority + interface number) wins.

In this case Switch 4 has a lower cost to root (2) than Switch 1 (4), so it wins the designated port election on that segment.

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