Interior Routing Protocols 

Interior Routing Protocols or IGPs operate within autonomous systems. The following sections provide brief descriptions of several IGPs that are currently popular in TCP/IP networks.

RIP

A discussion of routing protocols within an IP environment must begin with the Routing Information Protocol (RIP). RIP was developed by Xerox Corporation in the early 1980s for use in Xerox Network Systems (XNS) networks. Today, many PC networks use routing protocols based on RIP.

RIP works well in small environments but has serious limitations when used in larger internetworks. For example, RIP limits the number of router hops between any two hosts in an internet to 16. RIP is also slow to converge, meaning that it takes a relatively long time for network changes to become known to all routers. Finally, RIP determines the best path through an internet by looking only at the number of hops between the two end nodes. This technique ignores differences in line speed, line utilization, and all other metrics, many of which can be important factors in choosing the best path between two nodes. For this reason, many companies with large internets are migrating away from RIP to more sophisticated routing protocols.

IGRP

With the creation of the Interior Gateway Routing Protocol (IGRP) in the early 1980s, Cisco Systems was the first company to solve the problems associated with using RIP to route datagrams between interior routers. IGRP determines the best path through an internet by examining the bandwidth and delay of the networks between routers. IGRP converges faster than RIP, thereby avoiding the routing loops caused by disagreement over the next routing hop to be taken. Further, IGRP does not share RIP's hop count limitation. As a result of these and other improvements over RIP, IGRP enabled many large, complex, topologically diverse internetworks to be deployed.

Cisco has recently enhanced IGRP to handle the increasingly large, mission-critical networks being designed today. This new version of IGRP is called Enhanced IGRP. Enhanced IGRP combines the ease of use of traditional distance vector routing protocols with the fast rerouting capabilities of the newer link state routing protocols.

Enhanced IGRP consumes significantly less bandwidth than IGRP because it is able to limit the exchange of routing information to include only the changed information. In addition, Enhanced IGRP is capable of handling AppleTalk and Novell IPX routing information, as well as IP routing information.

OSPF

OSPF was developed by the Internet Engineering Task Force (IETF) as a replacement for RIP. OSPF is based on work started by John McQuillan in the late 1970s and continued by Radia Perlman and Digital Equipment Corporation (DEC) in the mid-1980s. Every major IP routing vendor supports OSPF.

OSPF is an intradomain, link state, hierarchical routing protocol. OSPF supports hierarchical routing within an autonomous system. Autonomous systems can be divided into routing areas. A routing area is typically a collection of one or more subnets that are closely related. All areas must connect to the backbone area.

OSPF provides fast rerouting and supports variable length subnet masks.

Integrated IS-IS

ISO 10589 (IS-IS) is an intradomain, link state, hierarchical routing protocol used as the DECnet Phase V routing algorithm. It is similar in many ways to OSPF. IS-IS can operate over a variety of subnetworks, including broadcast LANs, WANs, and point-to-point links.

Integrated IS-IS is an implementation of IS-IS for more than just OSI protocols. Today, Integrated IS-IS supports both OSI and IP protocols.

Like all integrated routing protocols, Integrated IS-IS calls for all routers to run a single routing algorithm. Link state advertisements sent by routers running Integrated IS-IS include all destinations running either IP or OSI network-layer protocols. Protocols such as ARP and ICMP for IP and End System-to-Intermediate System (ES-IS) for OSI must still be supported by routers running Integrated IS-IS.

Exterior Routing Protocols

EGPs provide routing between autonomous systems. The two most popular EGPs in the TCP/IP community are discussed in this section.

EGP

The first widespread exterior routing protocol was the Exterior Gateway Protocol. EGP provides dynamic connectivity but assumes that all autonomous systems are connected in a tree topology. This was true in the early Internet but is no longer true.

Although EGP is a dynamic routing protocol, it uses a very simple design. It does not use metrics and therefore cannot make true intelligent routing decisions. EGP routing updates contain network reachability information. In other words, they specify that certain networks are reachable through certain routers. Because of its limitations with regard to today's complex internetworks, EGP is being phased out in favor of routing protocols such as BGP.

BGP

BGP represents an attempt to address the most serious of EGP's problems. Like EGP, BGP is an interdomain routing protocol created for use in the Internet core routers. Unlike EGP, BGP was designed to prevent routing loops in arbitrary topologies and to allow policy-based route selection.

Important links to routing (courtesy: Kripa Kambli ) 

http://gaia.cs.umass.edu/kurose/network/algor/algor.htm

http://www.cis.ohio-state.edu/%7Egurari/course/cis677/cis677Se32.html#QQ1-53-96