GATE | GATE-CS-2006 | Question 82

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.

cs200682

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? [2 marks]
(A) B1, B5, B3, B4, B2
(B) B1, B3, B5, B2, B4
(C) B1, B5, B2, B3, B4
(D) B1, B3, B4, B5, B2


Answer: (A)

Explanation: Spanning tree is



                B1
                /  \
              /      \
             B5      B3
                     /  \
                   /     \
                  B4     B2 

Note that B4 and B2 are connected through B3 (Not B5) because B3 has lower serial number than B5.

One DFS traversal of tree is B1 B5 B3 B4 B2

Hence Option A is the Answer


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