Skip to main content

Routing Protocol

A routing protocol specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network. Routing algorithms determine the specific choice of route. Each router has a priori knowledge only of networks attached to it directly. A routing protocol shares this information first among immediate neighbors, and then throughout the network. This way, routers gain knowledge of the topology of the network.

Although there are many types of routing protocols, three major classes are in widespread use on IP networks:

Please notice that the term "Exterior gateway protocol" has two meanings. It could mean a category of protocols used to exchange routing information between autonomous systems (see: exterior gateway protocol). It could also mean a specific RFC-described protocol (see: Exterior Gateway Protocol).

Many routing protocols are defined in documents called RFCs.

Some versions of the Open System Interconnection (OSI) networking model distinguish routing protocols in a special sublayer of the Network Layer (Layer 3).

The specific characteristics of routing protocols include the manner in which they avoid routing loops, the manner in which they select preferred routes, using information about hop costs, the time they require to reach routing convergence, their scalability, and other factors.

OSI layer designation

Routing protocols, according to the OSI routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:

  • IS-IS runs on the data link layer (Layer 2)
  • Open Shortest Path First (OSPF) is encapsulated in IP, but runs only on the IPv4 subnet, while the IPv6 version runs on the link using only link-local addressing.
  • IGRP, and EIGRP are directly encapsulated in IP. EIGRP uses its own reliable transmission mechanism, while IGRP assumed an unreliable transport.
  • RIP runs over UDP
  • BGP runs over TCP

Interior gateway protocols

Interior gateway protocols (IGPs) exchange routing information within a single routing domain. Examples of IGPs include:

Exterior gateway protocols

Exterior gateway protocols exchange routing information between autonomous systems. Examples include:

Routing software

Many software implementations exist for most of the common routing protocols. Examples of open-source applications are Bird Internet routing daemon, Quagga, GNU Zebra, OpenBGPD, OpenOSPFD, and XORP.

Routed protocols

Some network certification courses distinguish between routing protocols and routed protocols. A routed protocol is used to deliver application traffic. It provides appropriate addressing information in its Internet Layer (Network Layer) addressing to allow a packet to be forwarded from one network to another.

Examples of routed protocols are the Internet Protocol (IP) and Internetwork Packet Exchange (IPX).