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A router receives a packet from a network and passes it to another network based on cost metric.

Today, an internet can be so large that one routing protocol cannot handle the task of updating routing tables of all routers. For this reason, an internet is divided into autonomous systems.
An Autonomous System (AS) is a group of networks and routers under the authority of a single administration. Routing inside an autonomous system is referred to as internal routing. Routing between autonomous systems is referred to as exterior routing.
Each autonomous system can choose an interior routing protocol to handle routing inside the autonomous system. However, only one exterior routing protocol is usually chosen to handle routing between autonomous systems.

Routing Information Protocol (RIP)
It treats each network as equal. The cost of passing through each network is the same, it is one hop count.
It is internal routing protocol. It is based on distance vector routing, using Bellman-Ford algorithm.

Distance Vector Routing:
1. Sharing knowledge about the entire autonomous system.
2. Sharing only with neighbors.
3. Sharing at regular intervals.

Open Shortest Path First (OSPF)
It allow the administrator to assign cost for passing through  a network based on the type of service required. For max throughput a satellite link is better. For min delay, a fibre optic line is better.
It allows each router to have several routing tabled based on the required type of service.

Link State Routing:
1. Sharing knowledge about the neighborhood.
2. Sharing with every other router.
3. Sharing when there is a change.

Border Gateway Protocol (BGP)
The criterion is the policy, which can be set by the administrator.