#CCNA Fast Start: a Spanning Tree Question

By certskills July 26, 2013 09:51

STP questions on the #CCNA exam require that you practice the rules for choosing the root switch, root port, and designated ports. If oyu ignore some of those pesky tiebreakers, the rules can be pretty straightforward. Often times, people understand STP basics, but fail to connect those basics with the various bits of show command output. This post gives you some output from a switch, with a multi-choice question for practice. Enjoy!

(Wendell – #213.)

The Question

The output comes from a switch named Fred. The output has to do with STP for VLAN 10 only. For this question, know that all switches in VLAN 10 make all STP choices without using any tiebreakers whatsoever. Which of the answers are true? (Note that while the real exam tells you the number of correct answers, this one does not!)

a)     Fred is the root switch in VLAN 10

b)     No other switches sit between Fred and the root switch

c)     Fred has no ports in VLAN 10 that are in an STP blocking state

d)     The switch connected to Fred’s G0/1 port could have a root cost of 3




Answers next post. Enjoy!

More Practice Questions:

The CCNA Skills blog has several more STP puzzles that have been posted in years past – make sure and check those out if you want more. For more questions on a large variety of topics:

#CCNA Fast Start: Spanning Tree
Answer to the #CCNA STP Question
By certskills July 26, 2013 09:51
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  1. Happy June 6, 00:54

    Hi Wendell,

    in the ROOT ID section of the show output the Root Cost is shown as 5. But in the answer to this question it is written that it is 4. I thought the root cost can be either 2,4,19,100, but not 5.
    Please correct me if i am wrong.

    Thanks inadvance!

    Reply to this comment
  2. Punya Atma August 30, 05:24

    A), and C).
    Fred, and, another switch, have same priorities, and, so the tiebreaker here is the MAC address. Fred’s MAC Address is lower than the other switch’s MAC. Therefore Fred selected as the root bridge.

    As Fred is the root bridge its other ports selected as designated ports, but not as blocking ports.

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