DISTANCE VECTOR ROUTING

A distance-vector routing algorithm, also called a Bellman-Ford algorithm is one where routes are selected based on the distance between networks. The distance metric is something simple-usually the number of hops, or routers, between them.

Routers using this type of protocol maintain information about the distance to all known networks in a table. They regularly send that table to each router they immediately connect with (their neighbors or peers). These routers then update their tables and send those tables to their neighbors. This causes distance information to propagate across the internetwork, so that eventually, each router obtains distance information about all networks on the internetwork.

Distance Vector Algorithm

1. A router transmits its distance vector to each of its neighbors in a routing packet.

2. Each router receives and saves the most recently received distance vector from each of its neighbors.

3. A router recalculates its distance vector when :

• It receives a distance vector from a neighbor containing different information than before.
• It discovers that a link to a neighbor has gone down.

The Distance Vactor calculation is based on minimizing the cost to each destination.

Example -

Initial Distances Stored at Each Router

Information Stored on RoutersDistance to Reach Other Routers
R1R2R3R4R5R6
R105??24
R250?3?1
R3??01?1
R4?310??
R52???03
R6411?30

We can represent each node's knowledge about the distances to all other nodes as a table like the one given above.

1. Every node sends a message to its directly connected neighbors containing its personal list of distance.
For example, R3 sends its information to its neighbors R4 and R6.

2. If any of the recipients of the information from R3 find that R3 is advertising a path shorter than the one they currently know about, they update their list to give the new path length and note that they should send packets for that destination through R3.
For example, router R6 learns from R3 that router R4 can be reached at a cost of 1; R6 also knows it can reach R3 at a cost of 1, so it adds these to get the cost of reaching R4 by means of R3. R6 records that it can reach R4 at a cost of 2 by going through R3.

3. After every router has exchanged a few updates with its directly connected neighbors, all routerss will know the least-cost path to all the other routers.

4. In addition to updating their list of distances when they receive updates, the nodes need to keep track of which node told them about the path that they used to calculate the cost, so that they can create their forwarding table.
For example, R6 knows that it was R3 who said "I can reach R4 in one hop" and so R6 puts an entry in its table that says "To reach R4, use the link to R3".

Maximum 1-hop paths

Table for R1

DestinationCostHop
R10R1
R25R2
R3-
R4-
R52R5
R64R6

Table for R2

DestinationCostHop
R15R1
R20R2
R3-
R43R4
R5-
R61R6

Table for R3

DestinationCostHop
R1-
R2-
R30R3
R41R4
R5-
R61R6

Table for R4

DestinationCostHop
R1-
R23R2
R31R3
R40R4
R5-
R6-

Table for R5

DestinationCostHop
R12R1
R2-
R3-
R4-
R50R5
R63R6

Table for R6

DestinationCostHop
R14R1
R21R2
R31R3
R4-
R53R5
R60R6

Maximum 2-hop paths

Table for R1

DestinationCostHop
R10R1
R25R2
R35R6
R48R2
R52R5
R64R6

Table for R2

DestinationCostHop
R15R1
R20R2
R32R6
R43R4
R54R6
R61R6

Table for R3

DestinationCostHop
R15R6
R22R6
R30R3
R41R4
R54R6
R61R6

Table for R4

DestinationCostHop
R18R2
R23R2
R31R3
R40R4
R5-
R62R3

Table for R5

DestinationCostHop
R12R1
R24R6
R34R6
R4-
R50R5
R63R6

Table for R6

DestinationCostHop
R14R1
R21R2
R31R3
R42R3
R53R5
R60R6

Maximum 3-hop paths

Table for R1

DestinationCostHop
R10R1
R25R2
R35R6
R45R6
R52R5
R64R6

Table for R2

DestinationCostHop
R15R1
R20R2
R32R6
R43R4
R54R6
R61R6

Table for R3

DestinationCostHop
R15R6
R22R6
R30R3
R41R4
R54R6
R61R6

Table for R4

DestinationCostHop
R15R3
R23R2
R31R3
R40R4
R55R3
R62R3

Table for R5

DestinationCostHop
R12R1
R24R6
R34R6
R45R6
R50R5
R63R6

Table for R6

DestinationCostHop
R14R1
R21R2
R31R3
R42R3
R53R5
R60R6

Router R1 update its routing-table for R4, also router R4 update its routing-table for R1.

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