22 - Internet Protocol Routing

Abstract

The task of routing in an IP network requires only that the router have a routing table and be capable of using it to make appropriate decisions about where to forward packets. However, as it is seen, IP routing protocols provide the mechanisms by which routers are able to exchange information about their links and update their routing tables to reflect changes reported by other routers. The difference between interior and exterior routing is an important one, inasmuch as interior routers usually have the simpler task of routing packets within smaller internets. When the interior router encounters a packet with a destination address outside its routing domain, it passes the packet along to its upstream default gateway. Exterior or backbone routers do not have the luxury of default gateways, and must be able to route packets to any destination. Routing protocols generally rely on either the distance-vector or link state approaches to distributing routing table data. Routing Information Protocol (RIP) is a simple distance-vector routing protocol, while Open Shortest Path First (OSPF) is a more complex routing protocol based on the link state algorithm. The chapter explains that routing is a task that has several different and sometimes contradictory measures of success — is it better to be fast at the risk of being inaccurate, or to be accurate at the risk of slowing network traffic? The attempt to somehow balance these strategies means that other problems can arise, including slow convergence and routing loops — most of which are remediable in some form.