Routing Protocols - Computer Network Tutorial

Routed versus Routing Protocols

Confusion often exists between the similar terms routing protocol and routed protocol. Routed protocols are any network protocol suite that provides enough information in its network layer address to allow a packet to direct user traffic. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. The Internet IP protocol and Novell’s IPX are examples of routed protocols. Other examples include DECnet, AppleTalk, Novell NetWare, Open Systems Interconnect (OSI), Banyan VINES, and Xerox Network System (XNS).

A routing protocol supports a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. Routing protocol messages do not carry end-user traffic from network to network. A routing protocol uses the routed protocol to pass information between routers. TCP/IP examples of routing protocols are the Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP).

Routing Protocol Evolutions

Distance Vector

RIP – Routing Information Protocol. The most common IGP in the Internet. RIP uses hop count as a routing metric.

IGRP – Interior Gateway Routing Protocol. IGP developed by Cisco to address the issues associated with routing in large, heterogeneous networks.

Link State

OSPF – Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the IS-IS protocol.

NLSP – NetWare Link Services Protocol. Link-state routing protocol based on IS-IS.

IS-IS – Intermediate System-to-Intermediate System. OSI link-state hierarchical routing protocol based on DECnet Phase V routing, whereby ISs (routers) exchange routing information based on a single metric, to determine network topology.

Hybrid

EIGRP – Enhanced Interior Gateway Routing Protocol. Advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency, and combines the advantages of link state protocols with those of distance vector protocols.

RIP and IGRP

RIP takes the path with the least number of hops, but does not account for the speed of the links. It only counts hops. The limitation of RIP is about 15 hops. This creates a scalability issue when routing in large, heterogeneous networks.

IGRP was developed by Cisco and works only with Cisco products (although it has been licensed to some other vendors). It accounts for the varying speeds of each link. Additionally, IRGP can handle 224 to 252 hops, depending on the IOS version. However, IGRP only supports IP.

OSPF and EIGRP

OSPF – Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the IS-IS protocol.

EIGRP – Enhanced Interior Gateway Routing Protocol. Advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency, and combines the advantages of link state protocols with those of distance vector protocols.