Networks and Protocols
Jacobs University Bremen
Dr. Jürgen Schönwälder
Course: 320301
Date: 2008-10-20
Time: 10 min.
Quiz Sheet #3
Problem 3.1: distance vector vs. link state routing
(2+1 = 3 points)
a) Briefly explain the major difference between distance-vector routing protocols and link-state
routing protocols.
b) What is the major advantage of a path-vector routing protocol over a distance-vector routing
protocol?
Solution:
a) A distance-vector routing protocol periodically exchanges distance vectors between neighbouring routers. The elements of a distance vector include the destination and the distance to the
destination. A router forwards traffic towards via the neighbour which has announced the
shortest distance to a given destination. The underlying routing algorithm is known as the
Bellmann-Ford algorithm. Distance vector protocols may suffer from the “count-to-infinity”
problem on link or node failures.
A link-state routing protocol propagates the topology of the network (the link-state) to all
routers. Every router then independently calculates the best next hop to every possible
destination in the network, usually using Dijkstra’s algorithm.
b) In contrast to distance-vector protocols, path-vector protocols carry information about the
whole path used to reach a certain destination. This additional information can be used
to detect loops and this helps to avoid the “count-to-infinity” problem that can occur with
distance-vector protocols.
Problem 3.2: distance-vector routing
(8 points)
B
A
C
D
Consider the above topology. Compute the shortest paths using the distributed Bellman-Ford
(distance-vector) algorithm. Assume that the network has just been initialized and that distance
vectors are exchanged in rounds periodically. After each round, show the routing information
available at each node.
Round 0:
A
Dst
Link
Cost
B
Dst
Link
Cost
C
Dst
Link
Cost
D
Dst
Link
Cost
Link
Cost
B
Dst
Link
Cost
C
Dst
Link
Cost
D
Dst
Link
Cost
Link
Cost
B
Dst
Link
Cost
C
Dst
Link
Cost
D
Dst
Link
Cost
Link
A-B
Cost
1
B
Dst
A
C
D
Link
A-B
B-C
B-D
Cost
1
1
1
C
Dst
B
D
Link
B-C
C-D
Cost
1
1
D
Dst
B
C
Link
B-D
C-D
Cost
1
1
Link
A-B
A-B
A-B
Cost
1
2
2
B
Dst
A
C
D
Link
A-B
B-C
B-D
Cost
1
1
1
C
Dst
A
B
D
Link
B-C
B-C
C-D
Cost
2
1
1
D
Dst
A
B
C
Link
B-D
B-D
C-D
Cost
2
1
1
Round 1:
A
Dst
Round 2:
A
Dst
Solution:
Round 0:
A
Dst
B
Round 1:
A
Dst
B
C
D
Round 2:
There are no further changes since the maximum path length between any two nodes is 2.