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

S1#show spanning-tree vlan 2 
Spanning tree enabled protocol ieee
Root ID    Priority    32770  
           Address     0019.e859.5380 
           This bridge is the root 
           Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec 

Bridge ID Priority     32770 (priority 32768 sys-id-ext 2)
          Address      0019.e859.5380
          Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
          Aging Time   15 sec 

! The rest of the output has been omitted


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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Avinash Kumar
Avinash Kumar
July 17, 2015 2:47 pm

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.


July 18, 2015 7:19 am

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 ).

June 6, 2017 1:05 am

Hi Wendell,

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


Gabriel Moran
Gabriel Moran
July 26, 2017 1:12 am

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?

August 29, 2017 2:26 am

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

Punya Atma
August 24, 2021 1:49 am

Answers:- A, D, and, E.

Punya Atma
August 24, 2021 2:14 am

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.

Punya Atma
August 29, 2021 5:55 am

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.

April 7, 2022 4:11 pm

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

Wourya Jawaheri
Wourya Jawaheri
Reply to  certskills
May 16, 2022 1:36 am

thank you Wendell, it is very clear now