STP question

Unanswered Question
Oct 11th, 2007

I'm taking the CCNA exam, again, tomorrow. Last time I got hung up on a question asking which switch is the root bridge. I was able to find the MAC of the root bridge, but don't know which switch in the diagram that MAC corresponds to. They all had generic names like SW1 and SW2. Also, I can't tell what port each switch is on.

Any suggestions?

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ibrahimkhalil Thu, 10/11/2007 - 16:30

If you don't know the answer, don't waste your time to hang on that question. Move on to the next question. In order to pass you no need to answer 100% or no need to get 100% score. If you waste time, you might not have enough time to go through rest of the questions.

bizdave123 Thu, 10/11/2007 - 16:36

You're missing the point. I got a 821 on the last test. The STP question was one that I know I need help on. If you can't help, keep your lecture.

nelsonewang Fri, 10/19/2007 - 01:47


this is simple. If you know the root bridge port as you claim, then use the command " show cdp neighbor" to get the device associated to that port. I had the same question when i was taking my ccna on the 13 ot October 2007.

I hope this will help you.

Good luck and God bless you


bizdave123 Fri, 10/19/2007 - 06:30

Unfortunatly, all I know if the Root Bridges MAC. I don't recall if I saw the root port. But thanks for the feedback. I got an 821 last time. I hope to do better this time.


alanc3141592654 Thu, 10/11/2007 - 19:48

hey there.

Was this a simulation question?

Did the diagram have the bandwidth of the links between all the switches?

bizdave123 Thu, 10/11/2007 - 20:18

Sim, yes. I only had one connection to a switch. I was able to show spanning tree to see what the root bridge MAC was, but don't know how to associate that with the name of one of the switches. The question gave generic names like SW1 and SW2. How do I connect the MAC to a switch name?

paul.matthews Fri, 10/12/2007 - 01:59

Please note I am talking real world, not exam sim.

There is no direct name to address mapping available. You may be able to locate using CDP. You may also benefit from looking at what port roles mean - designated, root etc.

BTW, the advice about not wasting time on a question was good advice, and did not warrant the response you gave it.

Kevin Dorrell Fri, 10/12/2007 - 02:08

There are a few leads you could have followed, depending on which commands were available on the SIM.

Did you know which switch you were connected to already? How many switches were in the topology?

Was the bridge id the same as the root id? If so, you are already talking to the root.

When you do a show spanning-tree, what was your root cost? If it was only 4, then you were probably only one hop away, so just follow the root port. If you don't know what is connected to the root port, try show cdp neighbor.

Did you know the bandwidth of the links? If so, you could work out some likely candidates from the root cost and the cost of each link (which you have to calculate).

I think the question was trying to test familiarity with Spanning-Tree as well as specific knowledge.

Kevin Dorrell


bizdave123 Fri, 10/12/2007 - 05:18

No bandwidths given. I believe I was connected to SW3. It had a different BID than the root bridge. I tried a few CDP commands but none seemed to be available.

I've been reading and rereading about STP and CDP in the books I have.

I guess I'll just have to fumble through, again. Thanks for your help.

paul.matthews Fri, 10/12/2007 - 06:38

Did a sh span tell you which was the root port? any designated ports? any blocked ports?

I am trying not to give too much away here, as I suspect that one of those port states would tell you where the root bridge is, and it is better that you understand than you know the answer IYSWIM.

There will almost certainly have been enough info available to work it out, providing you understand it.

I don't know this exam, but if you can review questions, any you struggle on mark or whatever and spend the time on it later - coming back at the end when you have 15 mins available means you will probably spot the clue in about 20 seconds, where looking at it as the third question, you struggle and waste 20 mins.

bizdave123 Fri, 10/12/2007 - 07:23

For starters, once you hit the "next" button, you cannot go back to previous questions.

The problem is that the diagram doesn't show what port the various switches are connected to. So, even if I know which is the root port, I still don't know which switch is connected to it.

Thanks for responding, though.

benson9010 Tue, 10/16/2007 - 12:34

If you do a show mac-address-table you can look to see if the mac address of the root bridge is in the table, if it is in the table(most likely not) then you can do a show cdp neighbor detail to get the name of the switch(root bridge) associated with the mac-address, if the mac-address of the root bridge is not in the table, by process of elimination you can track down the root bridge by checking the mac-addresses in the table to their device id in show cdp neighbor.

bizdave123 Tue, 10/16/2007 - 12:47

That makes sense. I was able to find the mac, so the show cdp neighbor sounds like it will do the trick. I need to lookup that command to verify what the output shows, but it sounds promising.

I've been layed off for a while now and have been studying without the ability to access much equipment. It's been a challenge.

Thanks benson.

cyphur353 Sun, 10/21/2007 - 05:07

I suggest downloading Packet Tracer from Cisco - that will at least allow you to play with some of the equipment. Since it's just the CCNA - it should cover most of what you need.

Please do not take offense from this remark - but it sounds like you need to work on more than just your comprehension of STP. Getting some hands on time with the equipment - even if it's just a simulator, will help you cement your ability to navigate the IOS and how to discover the necessary information to answer the questions on the exam.

I saw a similar question when I took my CCNA in 2005, and they provided you all the means and information to get the correct answer.

Best of luck with your next attempt!

vonschmieder Mon, 10/22/2007 - 11:09

I had the same question and I used the "show cdp neighbor detail" command which on the sim showed the switch priorities for the connected switches. One of them had a priority of 4096...

Yes I had that question too in the exam, but could not answer the question at that time. It need two three commands to figure out root bridge. You have to use Show spanning-tree, look at the root port then follow its port name, then use show cdp command to see which devide connect to this port. cdp will give you device name that is connected to that particular port.

By the way, whole simulation was similar to network but only you have access to switch Sw-AC3 via PC connected to it for configuration, No other configuration access. And ofcourse router subinterface IP number and corresponding VLAN numbers.

now I am trying to built and practice but till far could not get right.


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