STP Puzzle 3

Who will be elected root in this latest #CCNA STP Puzzle? I just voted, so it made me think about doing another STP puzzle series. The first step: which switch becomes root?

Big picture: Today’s post gives you part of the puzzle. Next post reveals what you could have derived from today’s post, and then it gives more detail – and asks you to take the next step to figure out what’s going on. The goal at each step: decide all STP facts you can about the LAN.

If you’ve not yet learned about STP, this exercise won’t help much. But you can go read more in my ICND2 book. Then try out the exercise!

Related posts:

• All STP Puzzles in the blog
• Overview of how to approach these STP puzzles
• Link to the ICND2 book (to learn more STP)

STP Puzzle Rules – Skip if You Know the Drill

For each puzzle, you will be given a problem statement that includes the following kinds of facts. For any facts that the problem statement does not tell you, figure out everything else on the list.

• Switch Bridge ID (BID)
• Switch priority (a part of the bridge ID)
• Root ports
• Designated ports
• Blocking ports
• Per-VLAN port cost
• Per-VLAN port priority
• Notes about a switch using default settings

Some rules:

1. The puzzles use CCNA-level concepts only.
2. The problem lists partial information, so you may not be able to determine all STP facts. Part of your job is to figure out what you cannot tell from the information given.
3. Unless otherwise stated, assume that the problem relates to the STP topology for VLAN 1
4. Unless otherwise stated, assume all switch-to-switch links are up and working physically, are performing VLAN trunking in the chosen VLAN. That is, the trunking state is not preventing STP from using the link.
5. Do *not* assume that the switches use default configuration. Just because the puzzle is silent about some parameter does not mean it has a default value.
6. If you have questions or comments, make sure and list you reasoning that leads up to the question or point.

STP Puzzle 3 – Problem Statement

Figure 1 shows all the initial facts revealed for the first round of this puzzle. It shows the topology, plus a few root ports (RP) and designated ports (DP). As always, identifying the root switch is the important first step, because without that knowledge, you cannot figure out the other port states. And everything sits inside one VLAN (call it VLAN 2 for the sake of discussion.)

Figure 1 – Given Information, STP Puzzle 3

That’s it. Ask questions on the process here if you have them. The next post will explain what I think can be discovered with the information supplied so far, and it will list more information, so you can take the next step. Enjoy!