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CSP LabBy Educator Today

Pilot · Lab P08 · 45 minutes

Testing a resilient network

Using the same seven-link network as P07, fail links on purpose and predict, before checking, whether S can still reach T. Find which single failures the network survives, which pair cuts S off completely, and whether adding one extra link would fix it.

Predict first

Write your predictions before you use the tool.

  1. If C–T fails, can S still reach T? If so, by which route?
  2. If S–A and S–B both fail, can S reach T? Would adding a B–T link help?
  3. Which two-link failure near T disconnects T, and would a B–T link repair it?

Practice tool

The network tool lets you mark any of the seven links as failed, optionally add an extra B–T link, and see the adjacency table of working links. After you record a prediction it reports whether S can reach T and shows one surviving path. It is a deterministic model: it sends no traffic and probes no real network.

Fault tolerance · S to T in a fixed seven-link network

Fail links
Adjacency table (working links)
SA, B
AC, D, S
BC, S
CA, B, T
DA, T
TC, D
Predict: can S still reach T?

Make a prediction, then check.

Failed links: none

Starter

Network links · Data
S–A, S–B, A–C, B–C, C–T, A–D, D–T
Adjacency table (all links working) · Data
S: A, BA: C, D, SB: C, SC: A, B, TD: A, TT: C, D

Worked case

Worked failure checks
failed linksS reaches T?surviving path
C–TyesS → A → D → T
S–AyesS → B → C → T
S–A and S–Bnonone: S has no working links

Redundancy means there is more than one path, so one failure does not disconnect the endpoints. Every single-link failure in this network still leaves a path from S to T.

When S–A and S–B both fail, S has no working links at all. A new link elsewhere, such as B–T, cannot help, because nothing connects to S. Redundancy only helps where it adds paths around the failure.

Build

  1. For each of the seven links, predict, then fail it alone and check. Record the surviving path.
  2. Fail S–A and S–B together; predict, check, then add B–T and check again.
  3. Fail C–T and D–T together; predict, check, then add B–T and check again.
  4. Propose one new link that would let S survive losing both S–A and S–B, and say what failure it still would not survive.

Tests

Tests for this lab
InputExpectedWhy this test
Fail C–TConnected: S → A → D → TThe D route bypasses the failure.
Fail S–A and S–BDisconnectedS is isolated.
Fail S–A and S–B, add B–TStill disconnectedThe new link does not touch S.
Fail C–T and D–T, add B–TConnected: S → B → THere the new link bypasses the cut.

Explain and reflect

Explain

  1. Explain why adding B–T repairs one double failure but not the other, using the adjacency table.
  2. Explain what “fault tolerant” means for this network, and its limit.

Reflect

  1. Every link you add has a cost. Which single extra link gives the most protection here, and why?
  2. How is this model simpler than real Internet routing?

What this lab does not do

  • The network and the optional B–T link are fixed; you cannot add other links in the tool.
  • The tool shows one surviving path, not all of them.
  • No real network is tested, and nothing is saved.