Network-Layer Routing

Routing tasks are methods of finding the paths for packet from their sources to their destinations.

Routers are responsible mainly for implementing

routing algorithms.

Classification of Routing Algorithms

We can classify different routing Algorithms as

1.Static Routing and Dynamic Routing2.Least Cost Path and Non-Least Cost Path3.Intra Domain and Inter Domain Routing

Static routing performs routing decisions with preconfigured routes in the routing table, which

can be changed manually only by administrators. Static routes are normally

implemented in those situations where there is no need for route change in the future. Also, if a

failure happens in the network, it is unable to react immediately. e.g. shortest path, flooding

In dynamic routing, the state of the network is

learned through the communication of each router with its neighbors. Dynamic routing continuously updates its routing table with paths and their cost/metric e.g. distance

vector, link state

Routing

Static Routing ( shortest path

Algo , Flooding )

and

Dynamic Routing ( Distance vector ,

Link state )

Least Cost Path ( Dijkstras , Bellman

Ford )and

Non-Least Cost Path (Flood Routing ,

Deflection Routing )

Interior ( RIP , OSPF )and

Exterior ( B G P )

Flooding

Flooding requires no network information whatsoever. Every incoming packet to a node is sent out on every outgoing line except the one it arrived on. After the first transmission all the router which are 1 hop distance receive the packet.After second transmission all the routers which are at 2 hop distance receive the packet .

Limitation

Main limitation flooding is that it generates vast number of duplicate packets due to which bandwidth gets wasted . It is necessary to use suitable mechanism to overcome this limitation.

Solution1. Use hop-count :- a hop counter may be contained in the packet header, which is decremented at each hop, with the packet being discarded when the counter becomes zero.

2. Use Sequence Number :- Avoid forwarding of same packet second time .

hop-count=3

D

G

A

F

E

B

C=router

=link

X

31

1

11

1

1

1

Distance Vector Routing

Distance Vector Routing Algorithm

• In routing table , initially each router Initialize neighbors with known cost, others with infinity .

• Periodically send copy of routing table to neighbors• On reception of a table , if neighbors path to a destination plus

neighbor cost is better, then switch to better path

Example

Dest Cost Next

B 3 BC 1 CD -E 1 EF 1 FG -

Initial Table at A

Dest Cost Next

B 2 CC 1 CD 2 CE 1 EF 1 FG 2 F

Table at A after single iteration