Tag: Routing Algorithms

Distance Vector Routing Algorithm | Example

Computer Networks

Routing Algorithms-

 

  • Routing algorithms are meant for determining the routing of packets in a node.
  • Routing algorithms are classified as-

 

 

  1. Static Routing Algorithms
  2. Dynamic Routing Algorithms

 

In this article, we will discuss about distance vector routing.

 

Distance Vector Routing Algorithm-

 

Distance Vector Routing is a dynamic routing algorithm.

 

It works in the following steps-

 

Step-01:

 

Each router prepares its routing table. By their local knowledge. each router knows about-

  • All the routers present in the network
  • Distance to its neighboring routers

 

Step-02:

 

  • Each router exchanges its distance vector with its neighboring routers.
  • Each router prepares a new routing table using the distance vectors it has obtained from its neighbors.
  • This step is repeated for (n-2) times if there are n routers in the network.
  • After this, routing tables converge / become stable.

 

Distance Vector Routing Example-

 

Consider-

  • There is a network consisting of 4 routers.
  • The weights are mentioned on the edges.
  • Weights could be distances or costs or delays.

 

 

Step-01:

 

Each router prepares its routing table using its local knowledge.

Routing table prepared by each router is shown below-

 

At Router A-

 

Destination Distance Next Hop
A 0 A
B 2 B
C ∞ –
D 1 D

 

At Router B-

 

Destination Distance Next Hop
A 2 A
B 0 B
C 3 C
D 7 D

 

At Router C-

 

Destination Distance Next Hop
A ∞ –
B 3 B
C 0 C
D 11 D

 

At Router D-

 

Destination Distance Next Hop
A 1 A
B 7 B
C 11 C
D 0 D

 

Step-02:

 

  • Each router exchanges its distance vector obtained in Step-01 with its neighbors.
  • After exchanging the distance vectors, each router prepares a new routing table.

 

This is shown below-

 

At Router A-

 

  • Router A receives distance vectors from its neighbors B and D.
  • Router A prepares a new routing table as-

 

 

  • Cost of reaching destination B from router A = min { 2+0 , 1+7 } = 2 via B.
  • Cost of reaching destination C from router A = min { 2+3 , 1+11 } = 5 via B.
  • Cost of reaching destination D from router A = min { 2+7 , 1+0 } = 1 via D.

 

Explanation For Destination B

 

  • Router A can reach the destination router B via its neighbor B or neighbor D.
  • It chooses the path which gives the minimum cost.
  • Cost of reaching router B from router A via neighbor B = Cost (A→B) + Cost (B→B)= 2 + 0 = 2
  • Cost of reaching router B from router A via neighbor D = Cost (A→D) + Cost (D→B) = 1 + 7 = 8
  • Since the cost is minimum via neighbor B, so router A chooses the path via B.
  • It creates an entry (2, B) for destination B in its new routing table.
  • Similarly, we calculate the shortest path distance to each destination router at every router.

 

Thus, the new routing table at router A is-

 

Destination Distance Next Hop
A 0 A
B 2 B
C 5 B
D 1 D

 

At Router B-

 

  • Router B receives distance vectors from its neighbors A, C and D.
  • Router B prepares a new routing table as-

 

 

  • Cost of reaching destination A from router B = min { 2+0 , 3+∞ , 7+1 } = 2 via A.
  • Cost of reaching destination C from router B = min { 2+∞ , 3+0 , 7+11 } = 3 via C.
  • Cost of reaching destination D from router B = min { 2+1 , 3+11 , 7+0 } = 3 via A.

 

Thus, the new routing table at router B is-

 

Destination Distance Next Hop
A 2 A
B 0 B
C 3 C
D 3 A

 

At Router C-

 

  • Router C receives distance vectors from its neighbors B and D.
  • Router C prepares a new routing table as-

 

 

  • Cost of reaching destination A from router C = min { 3+2 , 11+1 } = 5 via B.
  • Cost of reaching destination B from router C = min { 3+0 , 11+7 } = 3 via B.
  • Cost of reaching destination D from router C = min { 3+7 , 11+0 } = 10 via B.

 

Thus, the new routing table at router C is-

 

Destination Distance Next Hop
A 5 B
B 3 B
C 0 C
D 10 B

 

At Router D-

 

  • Router D receives distance vectors from its neighbors A, B and C.
  • Router D prepares a new routing table as-

 

 

  • Cost of reaching destination A from router D = min { 1+0 , 7+2 , 11+∞ } = 1 via A.
  • Cost of reaching destination B from router D = min { 1+2 , 7+0 , 11+3 } = 3 via A.
  • Cost of reaching destination C from router D = min { 1+∞ , 7+3 , 11+0 } = 10 via B.

 

Thus, the new routing table at router D is-

 

Destination Distance Next Hop
A 1 A
B 3 A
C 10 B
D 0 D

 

Step-03:

 

  • Each router exchanges its distance vector obtained in Step-02 with its neighboring routers.
  • After exchanging the distance vectors, each router prepares a new routing table.

 

This is shown below-

 

At Router A-

 

  • Router A receives distance vectors from its neighbors B and D.
  • Router A prepares a new routing table as-

 

 

  • Cost of reaching destination B from router A = min { 2+0 , 1+3 } = 2 via B.
  • Cost of reaching destination C from router A = min { 2+3 , 1+10 } = 5 via B.
  • Cost of reaching destination D from router A = min { 2+3 , 1+0 } = 1 via D.

 

Thus, the new routing table at router A is-

 

Destination Distance Next Hop
A 0 A
B 2 B
C 5 B
D 1 D

 

At Router B-

 

  • Router B receives distance vectors from its neighbors A, C and D.
  • Router B prepares a new routing table as-

 

 

  • Cost of reaching destination A from router B = min { 2+0 , 3+5 , 3+1 } = 2 via A.
  • Cost of reaching destination C from router B = min { 2+5 , 3+0 , 3+10 } = 3 via C.
  • Cost of reaching destination D from router B = min { 2+1 , 3+10 , 3+0 } = 3 via A.

 

Thus, the new routing table at router B is-

 

Destination Distance Next Hop
A 2 A
B 0 B
C 3 C
D 3 A

 

At Router C-

 

  • Router C receives distance vectors from its neighbors B and D.
  • Router C prepares a new routing table as-

 

 

  • Cost of reaching destination A from router C = min { 3+2 , 10+1 } = 5 via B.
  • Cost of reaching destination B from router C = min { 3+0 , 10+3 } = 3 via B.
  • Cost of reaching destination D from router C = min { 3+3 , 10+0 } = 6 via B.

 

Thus, the new routing table at router C is-

 

Destination Distance Next Hop
A 5 B
B 3 B
C 0 C
D 6 B

 

At Router D-

 

  • Router D receives distance vectors from its neighbors A, B and C.
  • Router D prepares a new routing table as-

 

 

  • Cost of reaching destination A from router D = min { 1+0 , 3+2 , 10+5 } = 1 via A.
  • Cost of reaching destination B from router D = min { 1+2 , 3+0 , 10+3 } = 3 via A.
  • Cost of reaching destination C from router D = min { 1+5 , 3+3 , 10+0 } = 6 via A.

 

Thus, the new routing table at router D is-

 

Destination Distance Next Hop
A 1 A
B 3 A
C 6 A
D 0 D

 

These will be the final routing tables at each router.

 

Identifying Unused Links-

 

After routing tables converge (becomes stable),

  • Some of the links connecting the routers may never be used.
  • In the above example, we can identify the unused links as-

 

We have-

  • The value of next hop in the final routing table of router A suggests that only edges AB and AD are used.
  • The value of next hop in the final routing table of router B suggests that only edges BA and BC are used.
  • The value of next hop in the final routing table of router C suggests that only edge CB is used.
  • The value of next hop in the final routing table of router D suggests that only edge DA is used.

 

Thus, edges  BD and CD are never used.

 

Important Notes-

 

Note-01:

 

In Distance Vector Routing,

  • Only distance vectors are exchanged.
  • “Next hop”values are not exchanged.
  • This is because it results in exchanging the large amount of data which consumes more bandwidth.

 

Note-02:

 

While preparing a new routing table-

  • A router takes into consideration only the distance vectors it has obtained from its neighboring routers.
  • It does not take into consideration its old routing table.

 

Note-03:

 

The algorithm is called so because-

  • It involves exchanging of distance vectors between the routers.
  • Distance vector is nothing but an array of distances.

 

Note-04:

 

  • The algorithm keeps on repeating periodically and never stops.
  • This is to update the shortest path in case any link goes down or topology changes.

 

Note-05:

 

  • Routing tables are prepared total (n-1) times if there are n routers in the given network.
  • This is because shortest path between any 2 nodes contains at most n-1 edges if there are n nodes in the graph.

 

Note-06:

 

  • Distance Vector Routing suffers from count to infinity problem.
  • Distance Vector Routing uses UDP at transport layer.

 

Next Article- IP Address | Classes of IP Address

 

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