GATE | GATE-CS-2014-(Set-1) | Question 33

Consider the following three statements about link state and distance vector routing protocols, for a large network with 500 network nodes and 4000 links.

[S1] The computational overhead in link state protocols 
     is higher than in distance vector protocols.
[S2] A distance vector protocol (with split horizon) 
     avoids persistent routing loops, but not a link
     state protocol.
[S3] After a topology change, a link state protocol 
     will converge faster than a distance vector
     protocol.

Which one of the following is correct about S1, S2, and S3 ?
(A) S1, S2, and S3 are all true.
(B) S1, S2, and S3 are all false.
(C) S1 and S2 are true, but S3 is false
(D) S1 and S3 are true, but S2 is false


Answer: (D)

Explanation:

Link-state:
Every node collects complete graph structure
Each computes shortest paths from it
Each generates own routing table

Distance-vector
No one has copy of graph
Nodes construct their own tables iteratively
Each sends information about its table to neighbors 

Source: http://www.cs.cmu.edu/~srini/15-441/S05/lectures/10-Routing.ppt

[S1] The computational overhead in link state protocols 
     is higher than in distance vector protocols.
[S2] A distance vector protocol (with split horizon) 
     avoids persistent routing loops, but not a link
     state protocol.
[S3] After a topology change, a link state protocol 
     will converge faster than a distance vector
     protocol.

S1 is clearly true as in Link State all nodes compute shortest path for whole network graph.

S3 is also true as Distance Vector protocol has count to infinity problem and converges slower.

S2 is false. In distance vector protocol, split horizon with poison reverse reduces the chance of forming loops and uses a maximum number of hops to counter the ‘count-to-infinity’ problem. These measures avoid the formation of routing loops in some, but not all, cases

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