GATE2005-26

Question

In a network of LANs connected by bridges, packets are sent from one LAN to another through intermediate bridges. Since more than one path may exist between two LANs, packets may have to be routed through multiple bridges. Why is the spanning tree algorithm used for bridge-routing?

• A. For shortest path routing between LANs

• B. For avoiding loops in the routing paths

• C. For fault tolerance

• D. For minimizing collisions

Answer 1

The answer is (B).

Since, in a spanning tree, there is a unique path from a source to the destination, which avoids loops, since it is a tree, and contains all the nodes, since it is a spanning tree.

Comments

is A wrong?

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I guess the MST uses the shortest path to build the tree without forming the loop in the tree. Option A only deals with shortest path routing, which is i guess not appropriate as compared with option B which is more clear according to MST. If the option A was "For avoiding loops in the routing table using the shortest path" then this would have made somewhat sense

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We have something called Time to Live to avoid loops. Isn't it?

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TTL is in IP header (ie Network Layer) and bridges work on Data Link layer

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Yes I missed that. Thanks for reminding.

Answer 2

Option (b)

Ref : https://en.wikipedia.org/wiki/Spanning_Tree_Protocol

Comments

is this in syllabus of GATE18?

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@Skyby token ring and bridges are removed

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@Swami patil but this comes under routing

Answer 3

For avoiding loop we use spanning tree algorithm.

Answer 4

ans is b