Note

This program assumes you are using Packet Tracer 8.X where you have access to the span cost command.

What Is This?

A small program to help you do Comp. Networks Lab 4. The program runs a simplified STP simulation and brute forces a bunch of costs until it stumbles upon the desired blocked ports configuration. Despite using a naive brute force approach, this program should run pretty fast actually since we're only trying 3 different possible costs (4,16,32).

How to Use?

1. Make sure you have Python 3.XX installed.
2. Download input.txt, main.py, and stp_simulation.py
3. Configure input.txt to match your topology (i've already configured it for term 2430's lab test)
4. Configure the blocked ports in input.txt to match your question. The order matters here. "S6 (blocked) -> S4" means the port connected to S6 is blocked.

Note

The numbering of the switches in the PDF starts from 1. However, the numbering starts from 0 in Packet Tracer. I've decided to start from 0 instead of 1 because then you won't have to do much mental gymnastics when configuring. Just keep in mind that if S6->S4 is blocked in the PDF, you must type "S5 (blocked) -> S3" in input.txt.

5. Run main.py
6. You will be prompted to select the root bridge. If you leave this blank, it will follow the switch with the lowest number (S0). Use this for questions where the root bridge is S2 or something.

It couldn't find a solution

If you're 100% sure you typed in the topology and blocked ports correctly, then it's almost certain that the blocked port configuration is impossible.

It found a solution!

Great. Notice that the program gives you priorities as well as costs. You must configure each switch to follow the priorities listed. After you're done with that, you may configure the costs. Note that cost=4 is the default cost for a link in term 2430's topology. So, if everything is cost=4 but one is cost=16 then you only have to change the cost of that one link to 16.

Note

The script runs a simplified STP where link costs are symmetrical. However, this is not the case for real STP where the link costs are applied per switch. Therefore, if the script says the cost of S4 <-> S6 should be 32, you may need to try setting the cost for S4 and S6. As a general heuristic, apply the cost to the switch with the blocked port. For example, if the script states Link S1 <-> S3: cost=32, you must check the PDF and see which switch has the blocked port. If the blocked port is on S1, then you must configure the cost of 32 on S1, NOT ON S3.

I'm getting so many impossibles. That seems a bit odd?

Remember that meme where the teacher trolls the students by making every answer as (A) in the MCQ section? The emergence of a pattern seems off to us, like we're doing something wrong, but if the script says that a topology is impossible, it probably is. Of course, you're free to try and crack it yourself. Nothing's stopping you.

Example

The corresponding input.txt for this configuration would be:

Topology:
S0 -> S1
S0 -> S2
S1 -> S2
S1 -> S3
S2 -> S4
S3 -> S4
S3 -> S5
S4 -> S6
S5 -> S6

Desired Blocked Ports:
S1 (blocked) -> S2
S1 (blocked) -> S3
S3 (blocked) -> S5

Useful Commands

Here are some commands that may help you.

en -> conf t -> span vlan 1 prio {PRIORITY} - for configuring priority

en -> conf t -> int {PORT_NUM} -> span cost {COST} - for setting the costs