Question: STP and Choosing a Root Port

By certskills July 17, 2015 07:05

Here’s a new CCNA-level question to consider, this time on Spanning Tree Protocol. Before diving in, be warned, it’s meant for study prep, not for assessment. That is, I’m not attempting to make this question exactly like an exam question, but rather, make it useful for forcing you to think about what you learned, apply it in a new way, and really understand this stuff.

In the switched network shown in the figure, consider only the spanning tree for VLAN 2. The switches use some mechanism for STP that creates a separate STP topology per VLAN, for instance, PVST+. The example that follows shows a portion of the output from a show command on switch S1.

Figure 1: Question Topology, Four Switches

Example 1: SW1 Output


There are no other devices or links in the network. All the links physically work and are enabled (no shutdown). All the links use 802.1Q trunking. Using this description, answer the following questions:

1) Which of the following settings impact a switch’s choice of root port in VLAN 2, on at least one of the switches in this topology?

A) S2’s VLAN 2 STP cost on F0/1

B) S3’s VLAN 2 STP cost on F0/2

C) S4’s VLAN 2 port priority on F0/2

D) S2’s VLAN 2port priority on F0/3

E) S3’s VLAN 2 STP cost on F0/1

F) None of the above

ACL Drill Set 1
Answers: ACL Drill Set 1
By certskills July 17, 2015 07:05
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  1. Avinash Kumar July 17, 14:47

    Dear Sir,
    Option A – S2’s VLAN 2 STP cost on F0/1

    If we increase the cost to a value more than 38 , then S2’s Fa0/1 will no longer be the Root port for VLAN 2.


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  2. Samir July 18, 07:19

    Hi Mr Odom, i think answer is “A”

    P.S “B” and “E” options is the same ( S3’s VLAN 2 STP cost on F0/2 ).

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  3. Happy June 6, 01:05

    Hi Wendell,

    i think answers are A and D.
    As Samir noted, the answers B and E are same.


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    • certskills Author June 6, 10:14

      Thanks Happy!
      I changed Answer E to mention S3’s F0/1.
      The I believe A, B, and E are correct – check those figures in the middle of the explanation. Those those the interfaces whose costs, if changed, will impact the root port choice by both S2 and S3.
      But this is a fairly challenging exercise, and somewhat abstract!

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  4. Gabriel Moran July 26, 01:12

    Wendell, why is B considered in this scenario? Is it because if SW3 F0/1 fails then F0/2 port cost is considered? If that is the case, if SW3 F0/1 fails, it has no other choice than to forward out F0/2 and port cost wouldn’t be considered correct?

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  5. dell August 29, 02:26

    Hi Wendell , first I would like to say that i love all your books, i purchased most of your current books for ccna .im just confused with your icnd2 200-105 book,part 1,chapter 3,review question num 5 page 87, i am wondering why the answer is A (says local switch is the root)is included as correct answer. the question, ask us to examine the first 7 line of the output of “show spanning-tree”command, the command shows the protocol used(rstp)priority number,mac add,cost:15,port and hello timer. To my understanding , in a root switch ,cost should be 0, since no other cost to reach it self , and another there must be”this bridge is the root. I think the local switch is not the root,please correct me if a am wrong. thanks

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    • certskills Author August 29, 11:23

      Hi Dell,
      Thanks – glad you love the books!
      First, for some quick background, in case it’s helpful…
      We post the errata file for these books off the “Updates” tab on the web page for the book.
      That’s true for all Cisco Press books, and it’s collected mostly from reader feedback we receive per the instructions on page iii of each book under “Feedback”. So you can find all know errata (that is, errors that are meaningful; we don’t post the obvious typos) on the equivalent page for any Cisco Press book.

      That said, indeed, answer A is not correct. Note that the question appears on page 70 of the retail edition, not page 87 – maybe you’re reading the Academic Edition (which has different page numbers), and even has a different errata file with page numbers for that book). Anyway, if you look at the errata file, you’ll see where for page 71 (retail edition) it shows that the incorrect answer set of A, D should instead list only letter D as a correct answer.

      Hope this helps,

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  6. Punya Atma August 24, 01:49

    Answers:- A, D, and, E.

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  7. Punya Atma August 24, 02:14

    A). S2’s F0/1 Path cost to the root is lowest.

    D). I am sorry. This choice of answer is wrong, because here priorities of both switches to the root seem equal. In other words both answers, C), and, D), could be counted as correct.

    E). S3’s cost to the root on F0/1 is lowest.

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  8. Punya Atma August 29, 05:55

    Sorry again for the confusion caused by my earlier answers.

    The correct answers are, A), and, E), because of the lowest path costs to the root bridge S1, from F0/1 of S2, and F01 of S3.

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  9. Wourya April 7, 16:11

    Hi Wendell,
    I hope you doing well. could you tell me why the port connected to Endpoint is always has the DP role? BTW Ive studied the book but is wasnt clear me out

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    • certskills Author May 11, 09:24

      Thanks, getting better every week!
      Short version: yes. Longer version: On any link, the devices send their Hello BPDUs, which list their root cost. Lowest cost wins. On an edge port, with one switch and one endpoint, the endpoint does not use STP and does not send a Hello. So, the switch sends its Hello, receives none, always wins with lowest root cost, and always become DP. That role then tells the switch to migrate to a Forwarding state, which of course is needed for the port to operate and send/receive frames with the endpoint.
      Hope this helps,

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