The following subsections briefly introduce the RIP, IGRP, and EIGRP routing protocols. IGRP and EIGRP were developed to overcome the limitations of the original RIP design.

RIP?A Distance-Vector Routing Protocol (Bellman-Ford-Fulkerson)

RIP had been around for quite a while. As Jeff Doyle put it, "RIP is either unjustly maligned or undeservedly popular."[2] RIP comes in two flavors: RIPv1, which is classful, and RIPv2, which is classless. It supports multicast, broadcast, and (under certain conditions) unicast behavior. The most dominant constraint of both RIP versions is the hop-count limit of 15, which limits the network diameter of deployments.

RIP is a distance-vector protocol, designed for rather small networks, contiguous address blocks, and homogeneous data links. From the protocol point of view, RIPv1 and RIPv2 speakers and listeners can happily coexist; the necessary compatibility mechanisms work well. However, this usually introduces undesirable effects with regard to classful behavior and summarization. Therefore, the recommendation is not to use RIPv1 whenever possible or at least not mix RIPv1 and RIPv2. In addition, RIPv2 introduces authentication and prefix tagging and is based on multicast transmission facilitating and 520/udp as a transport vehicle.

Contrasting common beliefs that it is archaic and obsolete, RIP still plays an important role in system-integration scenarios. It is the least common denominator for Microsoft gateways and cheap appliance routers that lack support of more sophisticated dynamic routing protocols. Besides, in small LAN topologies, RIP certainly is up to the job, and system administrators can easily grasp its concepts and feel comfortable. Service providers can as well easily control RIP routes within customer virtual private networks (VPNs), while allowing customer premises equipment (CPE) to inject routes themselves.


The proprietary Cisco protocols IGRP and EIGRP were originally developed to overcome some of the limitations of RIP. IGRP has almost entirely disappeared and is not deployed anymore, although it is still supported.

EIGRP has evolved into quite a powerful and useful routing protocol including advanced features, the DUAL algorithm (Diffuse Update Algorithm), and a composite metric. EIGRP is the only routing protocol that supports non-IP network layer protocols such as AppleTalk and Internetwork Packet Exchange (IPX), with the notable exception of IS-IS, which natively supports IP and ISO CLNP (Connectionless Network Protocol).

Discussion of (E)IGRP goes beyond the scope of this book. Once again, I recommend Jeff Doyle's excellent two volumes of Routing TCP/IP for an in-depth introduction as well Ivan Pepelnjak's classic textbook EIGRP Network Design Solutions (Cisco Press, 2000).