GATE CSE 2005 | Question: 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

Comments

References : Tanenbaum 

References : Forouzan

 

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

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.

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Yeah you can think like, using the shortest path algos we can find the shortest path from source to destn and actually not requiring to have connected all the vertices(LANs) but in MST solution all the vertices are necessary to be connected and finding the shortest path. So definetly there it of no very use of using MST for shortest path finding between lans, But we definetly know that MSTs do not have the cycles and thats what we want in the routing paths.

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

Brides and switches works in Datalink layer where we don't have a TTL field to limit the hop count to avoid looping of packet like network layer. So to avoid looping of packet in a cycle we use spanning tree protocols to disable some links virtually to break the cycle so that packets don't loop forever.

so Option B is correct.

Answer 5

• The spanning tree algorithm is used for bridge-routing in LAN networks to avoid loops in the network topology.

• In a network of LANs connected by bridges, loops can cause broadcast storms, where packets are broadcast repeatedly and endlessly, clogging the network and causing traffic congestion.

• The spanning tree algorithm is used to construct a logical tree-like structure that includes all the LANs in the network and avoids loops. This logical structure is called the spanning tree.

• The algorithm determines which bridges should be designated as the root of the spanning tree and which links should be designated as forwarding links and which should be designated as blocking links.

• The algorithm ensures that there is only one path between any two LANs in the network, which avoids loops and eliminates the possibility of broadcast storms.