Consider the diagram shown below where a number of LANs are connected by…

2006

Consider the diagram shown below where a number of LANs are connected by (transparent) bridges. In order to avoid packets looping through circuits in the graph, the bridges organize themselves in a spanning tree. First, the root bridge is identified as the bridge with the least serial number. Next, the root sends out (one or more) data units to enable the setting up of the spanning tree of shortest paths from the root bridge to each bridge.
Each bridge identifies a port (the root port) through which it will forward frames to the root bridge. Port conflicts are always resolved in favour of the port with the lower index value. When there is a possibility of multiple bridges forwarding to the same LAN (but not through the root port), ties are broken as follows: bridges closest to the root get preference and between such bridges, the one with the lowest serial number is preferred.

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For the given connection of LANs by bridges, which one of the following choices represents the depth first traversal of the spanning tree of bridges?

Answer: A. B1, B5, B3, B4, B2 — Solution summary: Identify the root bridge: B1 is the root (lowest serial number). Each bridge selects a root port that is on the shortest path to B1; if…

  1. A.

    B1, B5, B3, B4, B2

  2. B.

    B1, B3, B5, B2, B4

  3. C.

    B1, B5, B2, B3, B4

  4. D.

    B1, B3, B4, B5, B2

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Correct answer: A

Solution summary:

  • Identify the root bridge: B1 is the root (lowest serial number).

  • Each bridge selects a root port that is on the shortest path to B1; if there is a tie between ports the port with the lower index is chosen.

  • For each LAN the designated bridge (the one that forwards toward the LAN) is the bridge closest to the root; if multiple bridges are equally close the bridge with the lower serial number is chosen.

Applying these rules to the given topology yields the following parent→children relationships in the spanning tree:

  • B1 → B5, B3

  • B3 → B4

  • B4 → B2

Perform a depth‑first traversal starting at the root (visit a bridge, then recursively visit its children in the order determined by the port/index tie‑break rules):

  1. B1

  2. B5

  3. B3

  4. B4

  5. B2

Therefore the correct depth‑first traversal of the spanning tree is: B1, B5, B3, B4, B2.

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