#CCNA Fast Start: a Spanning Tree Question
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:
- Look at the Questions tab in the CCENT Skills blog
- Look at the Questions tab in the CCNA Skills blog
- Use the practice tests that come with the printed version of the book
- Get additional exam banks, even more than the print book, with the Premium Edition of the Book, available only from the publisher
- Check out these STP puzzles here in the blog!
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!
I’m confused by this as well. Am I missing something? How do you get a cost of 5 on a Gig link?
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
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.
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
Mike,
On your first comment, if you’re having trouble finding the answer post, it’s at the link in text at the bottom of this page, but above the comments, “Answer to the CCNA STP Question”.
But, if you’re looking for the book content in the current book, yeah, you want chapters 9 and 10 of the CCNA 200-301 OCG. The great thing about blog posts that link to resources in current products is that it helps… while those products are current. The bad thing is that in a free site like this one, updating links for each product release is unlikely. (This post is from 2013, for instance with new books released in 2016 and 2019.) Still useful concepts, but some old product references. Sorry about that.
On your final comment, about the ACL posts, I’m not seeing that. But that may be immaterial now? Feel free to follow up.
Wendell