# STP stumbling block

Sep 20th, 2007

Ok, if you see a network drawn out in front of you, and it has, say, 10 switches total connecting to each other, you're only going to have ONE "red X", showing where the system will prevent switching loops. Since you have many switches, and each one has a segment with one end with a lower/higher mac than the other end, which segment is going to contain that "red X"?

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## Replies

avillalva Thu, 09/20/2007 - 18:41

The question is testing you knowledge of the spanning tree algorithm. Once you understand root bridge elections, root ports and designated ports you will understand where the red X goes : )

hope that helps,

Andres

paul.matthews Fri, 09/21/2007 - 00:10

Step 1 - find the root bridge.

Step 2 - look at link cost to get to root

step 3 - break the route that would have highest cost.

This is of course an over simplificationm as you also need to understand the tie-break and designated port methods.

EG you have four switches switch one is the root, switch two and three connect to one, the four connects to those two, so you have a square. one of the links between four and either two or three will be blocking, but how does it decide which link to block? Easy if one leg has 10M links and the other has 100, but if all links are the same then you need to know which gets picked.

1. With multiple equal costs back to root, which one is selected.

2. With multible bridhes/switches onto a common lan, which bridge/switch gets selected to be the forwarding port?

Paul.

crunchyhippo Fri, 09/21/2007 - 04:04

I *finally* figured it out. What clarified it for me was to realize that every SWITCH has a root port, and every SEGMENT has a designated port. This tells me which switches to look for what. It had never been explained to me this way. It was a combination of looking in Examcram for CCNA and Wikipedia under STP that turned the light on.

crunchyhippo Fri, 09/21/2007 - 14:30

I FINALLY figured it out after staring at it for several hours. Every SWITCH has a root port, and every SEGMENT has a designated port. This was really all I needed to understand, as this showed me what I needed to look at. Now it's a piece of cake. Such a simple thing...