An interior gateway protocol (IGP) is a type of protocol used for exchanging routing information between gateways (commonly routers) within an autonomous system (for example, a system of corporate local area networks). This routing information can then be used to route network-layer protocols like IP.
Interior gateway protocols can be divided into two categories: distance-vector routing protocols and link-state routing protocols. Specific examples of IGP protocols include Open Shortest Path First (OSPF), Routing Information Protocol (RIP) and Intermediate System to Intermediate System (IS-IS).
By contrast, exterior gateway protocols are used to exchange routing information between autonomous systems and rely on IGPs to resolve routes within an autonomous system.
Type of Interior gateway protocols
Distance-vector routing protocol
Distance-vector routing protocols use the Bellman–Ford algorithm. In these protocols, each router does not possess information about the full network topology. It advertises its distance value (DV) calculated to other routers and receives similar advertisements from other routers unless changes are done in local network or by neighbours (routers). Using these routing advertisements each router populates its routing table. In the next advertisement cycle, a router advertises updated information from its routing table. This process continues until the routing tables of each router converge to stable values.
Some of these protocols have the disadvantage of slow convergence.
Examples of distance-vector routing protocols:
- Routing Information Protocol (RIP)
- Routing Information Protocol Version 2 (RIPv2)
- Routing Information Protocol Next Generation (RIPng), an extension of RIP version 2 with support for IPv6
- Interior Gateway Routing Protocol (IGRP)
Link-state routing protocol
In link-state routing protocols, each router possesses information about the complete network topology. Each router then independently calculates the best next hop from it for every possible destination in the network using local information of the topology. The collection of best-next-hops forms the routing table.
This contrasts with distance-vector routing protocols, which work by having each node share its routing table with its neighbours. In a link-state protocol, the only information passed between the nodes is information used to construct the connectivity maps.
Examples of link-state routing protocols:
Hybrid routing protocol
Hybrid routing protocols have both the features of distance vector routing protocols and linked state routing protocols. One example is Enhanced Interior Gateway Routing Protocol (EIGRP).