Answer to the #CCNA STP Question

No muss, no fuss, let’s dig into STP for #CCNA with the explanation and answer to the previous post’s STP question! Complete with pretty pictures!

• • • Link to the question
    • Link to the brief topic introduction

(Wendell – #213.)

The Answer(s)

Answer(s): B, C

The Explanation

I organized the answers in the order shown on purpose so I could break down the explanation in the same order. Answers A and B build on each other, while C and D mostly stand alone. All require that you connect the show command output from the question to all those STP rules, so make sure and refer back to the question post!

Answer A (Incorrect)

The show spanning-tree vlan 10 command output from the original question gives you two ways to identify this answer as incorrect. First, the command lists two opening groups of messages: the first is about the root switch, and the second is about the local switch on which the command was entered. If the BID is different in these two message groups, then the local switch (Fred in this case) is not the root.

Second, note that the bottom of the output identifies port G0/2 as Fred’s RP (root port). Only non-root switches have an RP, so the fact that Fred has an RP rules out Fred as being the current root switch in VLAN 10.

Answer B (Correct)

Continuing to look at the command output, and adding to the analysis from answer A, we know:

• Fred is not the root
• Fred’s G0/2 is Fred’s root port

The show spanning-tree vlan 10 command output also lists Fred’s root cost, as well as the STP cost for each interface. In this case, the root cost is listed as 4 (first message grouping), and Fred’s G0/2 interface cost is listed as 4 (towards the end of the output). From these two facts, we can analyze as follows:

1. The root switch always has a cost 0 to reach itself.
2. The minimum STP interface cost is 1
3. Fred’s G0/2 cost is 4
4. If another switch sits between Fred and the root, Fred’s root cost must be at least 5 (Fred’s interface cost of 4, plus at least 1 from the interface cost of the switch between Fred and the root)

Therefore, no other switch could possibly be between Fred and the root. See figure 2 for a visual of the presumed switch between Fred and the root, with the blue items being the known items.

Figure 2: Minimum Root Cost for Fred if Another Switch Exists

Personally, I doubt Cisco would give you something this abstract on the real exam, but I though this answer would be kinda fun to think through with you. Ask questions if you have them!

Answer C (Correct)

Answer C again requires familiarity with the show spanning-tree vlan 10 command output. In this case, any ports currently in a blocking state would be listed at the bottom of the output, with the letters “BLK” under the heading “STS”. No ports are listed as blocking, making answer C correct.

Answer D (Incorrect)

While the analysis of answer C was simple, the analysis for answer D is pretty involved.

First, the question does not tell you anything about the topology of the LAN. What switch is connected to Fred’s G0/1 port? Literally, we know nothing about that switch – or even if there’s a switch connected to the other end of the cable – except what you can deduce from the command output on Fred.

So, what do we know?

1. Fred’s G0/1 acts as a designated port.
2. The question stem stated that no tiebreakers were used for picking root ports or designated ports.
3. Fred’s root cost = 4.

Based on 1 and 2, you can deduce that if a switch exists on the other end of the cable connected to Fred’s G0/1, that switch must have a root cost >4. Why? If such a switch existed (call it Barney), Fred would advertise its Hello with Fred claiming a root cost of 4. Barney would send a Hello, root cost = x. Fred won (with a lower number), and it wasn’t a tie. So, Barney’s root cost must be at least 5.

More Practice Questions:

This question is like those you get if when you buy the ICND2 200-101 Official Cert Guide. This blog also lists various practice questions as well. 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!