Static routing is used :
1. if network is small
2. if there is a single connection point to other networks
3. if there are no redundant routes
Dynamic routing is used :
if any one of above conditions fail.
Under dynamic routing, routers communicate to each other, informing each other about the networks, they are currently connected to. Routers communicate with other routers within an autonomous system via. interior gateway protocols (IGPs). Router from one autonomous system communicate with router in other autonomous systems via. exterior gateway protocols (EGPs).
Interior Gateway Protocol
Routing Information Protocol (RIP)
It is a distance vector protocol. i.e. messages sent by RIP contain 'a vector of distances' or 'hop counts' . Each router updates its routing table after receiving hop counts from its neighbour router.
Open Shortest Path First (OSPF)
It is a newer alternative to RIP. It is a link state protocol. It does not exchange vector distances with its neighbours. Each router tests status of its link with neighbour routers and prepares its routing table.
Exterior Gateway Protocol
Border Gateway Protocol (BGP)
Works with routers belonging to two different autonomous systems. It exchanges network reachability information. That is, the full path from one network to other.
