#CCNA Fast Start: a Spanning Tree Question

 In 200-301 V1 Ch09: Spanning Tree, 200-301 V1 Part 3: VLANs, STP, Q&A

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

 

Fred# show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778
             Address     1833.9d7b.0e80
             Cost        4
             Port        26 (GigabitEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     1833.9d7b.1380
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/12              Desg FWD 19        128.12   P2p
Gi0/1               Desg FWD 4         128.25   P2p
Gi0/2               Root FWD 4         128.26   P2p

 

 

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
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Happy

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!

DAKOTA

I’m confused by this as well. Am I missing something? How do you get a cost of 5 on a Gig link?

Wendell Odom

Dakota,
Well, Happy had asked about the text showing a root cost of 5. The post doesn’t show that today. This post is nine years old, so it could be that I changed it. But the root cost on the post here isn’t 5.
That said, you asked about port cost, I think. I think you may be restricting your thinking to default port costs. However, the port cost can be configured to any positive integer up to some ridiculously large number I don’t recall. So yeah, spanning-tree [vlan x] cost 5 would do the trick.
Wendell

Punya Atma

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.

Mike Hewitt

Hi Wendell,
I’m having trouble finding the answer post to this question post (#213). I found it in Vol 1, Part 3, Ch09 listing. I also noticed that the next 2 more recent posts refer to ACL posts at the end of those posts.
Thanks for your help!
Mike

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