STP Puzzle #1
This blog post is the first of what may be a whole new type. You can read all day about Spanning Tree Protocol (STP), and learn the theory, but that’s not enough. When you later try to apply STP concepts to a new topology, or to the same topology that has different STP settings, many people just need practice working through the concepts. That practice is very useful, and my ICND2 book has practice questions about STP. However, it seemed like this is a topic for which a few more practice problems could help.
I’ll do a longer background post on what I’m after here, but I wanted to go ahead and post a problem and see if I could get you folks thinking and get some feedback. If you want to think about STP, look below the fold!
Big Picture: Find the Root Switch, Root Ports, Designated Ports, and all Blocking Ports
The heading gives you most of your marching orders. This problem gives you some information, but not all information, about setting related to STP in the small switched network shown in Figure 1. Treat the information given as a puzzle, and see how much you can figure out about the topology. In particular, what can you determine for certain in regards to which switch is root? The root port on all non-root switches? The designated port on each link? And which ports block?
To list your answers, just mark each port in the figure as RP (root port), DP (designated port), or BL (blocking), and note the root switch. When you can, note each non-root switches cost to reach the root through its root port, which of course matters to the decision of which switch becomes the designated port on links.
- 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.
- Unless otherwise stated, assume that the problem relates to the STP topology for VLAN 1
- Do *not* assume that the switches use default configuration. That is, if a fact is not stated by the problem, it may be set to a default value, but it also may not e set to a default value.
- If you have questions or comments, make sure and list you reasoning that leads up to the question or point.
On that last rule, what I mean is this. If you believe it is impossible to know whether switch S1 or S3 is the root, but you know that it’s not switch S2 or S4, and you want to confirm that you cannot know for sure if S1 or S3 is root, then maybe start with why S2 and S4 are not root.
The switches use the topology as shown in Figure 1:
Examples 1 through 4 lists the facts supplied as part of the problem.
Example 1: S1 Facts
VLAN 1 Bridge ID: 32769:0020:1111:1111 Port F0/2 Cost: 8 Port F0/4 Cost: 8
Example 2: S2 Facts
Port F0/1 Cost: 4 Port F0/3 Cost: 19
Example 3: S3 Facts
Root Port: F0/1 Port Costs: All defaults
Example 4: S4 Facts
VLAN 1 Bridge ID: 28673:0020:4444:4444