Spanning Tree Questions

One of my students writes in with the following question:

***Keep in mind that the question below does not refer to an actual test question, rather a practice test question from a Cisco Certified Source.  GlobalConfig.net does not recommend the use of BrainDump, Cheating Sites or Pirated material***

Brandon, Can’t find this is any of my books from class or purchased books. I’m sure it’s on web sites but oh well These practice tests aren’t fair to the training and book information.   Need serious help with this spanning-tree question. If switch E is the ROOT Bridge then: What route will Host 1 take to get to server x. I don’t know if traffic always has to go to the root bridge?   I’ve messed around with this but the practice test doesn’t give priority, cost, or port numbers.   I understand all three versions of spanning-tree but this is a bit complicated. Not sure if a switch is the root bridge if everything has to go through it and how the other loops are handled. I know all ports on the root bridge become designated-forwarding. Least cost non-root bridge ports become Root ports.  But not sure on all the other ports.   Maybe you could list what ports would be R, D, and blocking.   The scenario is does host 1  go from 1-      A, E, C, D, and then to server X? 2-      A, B, C, D, and then to server X? 3-      A, D, and then to server X?   Thanks, Jeff

Now here is a look at the topology in question:

 

So where do we begin?  Well since this is an example of a test question and not all the information that you may use is not made available to use lets think about spanning-tree and what we do know about it.  Here are the fundamental rules of spanning tree.

1. One root bridge per network (vlan)
2. One root port per non-root switch
3. One designated port per segment

Now we do know that SwitchE is the root:

The scenario calls for us to determine which path Host 1 will take to communicate with Server X.

Now knowing that each switch gets a root port as long as they are not Switch E we could determine that Switch A would choose the link directly connected to Switch E as its root port.

Now finding the designated ports could be tricky because there isnt a lot of information given but lets look at it from the perspective of loops and fill in the blanks. First the Blue Loop:

You could assume that each switch that is not the root is going to find the best path back to the root. In this case the Green squares would represent root ports.

Now we can fill in the designated ports. Remember its 1 per segment.

Now we have a link between Switch A and Switch B that doesnt have a root port so one needs to be designated and the other side will be non-designated, or blocked. It doesnt matter in this case which one you choose. You dont have all the information so just pick one.

Now lets look at the yellow and orange links that indicate paths that are available for Switch D, and lets figure out how Switch D will reach the root. You have to assume it will choose the shortest path back to the root, assuming that all links are the same cost. The Problem with that logic is that both paths would be the same.

Normally we would use other information as the tie breaker, however this information is not given to us in the question. Lets go ahead and just choose one.

In this new topology you can see that Switch D will travel to Switch C to get to the Root. Now where does this leave us with the original question. How does Host 1 get to Server X. The answer for this example, the way spanning tree sits right now, is that the packet will travel from Host 1 to Switch A to Switch E to Switch C to Switch D to Server X.

Now the question is, will it actually do this? I would say that the question doesnt give adequate information. You can only assume and you know what happens when you assume. Keep in mind that we guessed what ports Switch D would choose as the root and blocked ports so the answer could have changed had we chosen different ports.

Best bet? Recreate it if you have the switches.

Alternative? Guess and move on.

While the exam is trying to test your knowledge of a topic area it is not trying to confuse you. By giving you too little information there may not be a right answer (which the test probably wouldn't do) so I personally would choose the best "wrong" answer and move on.

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