IGP (Interior Gateway Protocols)
The term IGP (Interior Gateway Protocol) is used to describe any routing protocol operating as a separate routing domain within an AS. IGPs learn about routes to networks that are internal to the AS, hence the name Interior.
Within an organization's network there may be one or more routing protocols (IGPs) keeping track of the routes to subnets within the AS. Routers running a single IGP (routing protocol) only share route information with other routers running the same routing protocol. Routers running more than one IGP, like RIP and OSPF, are participants in two separate routing domains. These routers are referred to as border routers, that is, they sit on the border between two IGP routing domains. Some examples of IGP protocols are: RIP, OSPF, and IGRP. Individual routing protocols will be discussed throughout this book.
Routing protocols that reside within an organization's AS are considered IGPs. Each IGP, consisting of subnets and connected routers, represents a separate routing domain.
IGPs are responsible for building and maintaining route information within a single Autonomous System (see Figure 3.16).
Figure 3.16 There are two IGPs (RIP and IGRP) running within a single Autonomous System. The router connecting the IGPs is a border router, running both RIP (on its lower interface) and IGRP (on its upper interface).
In Figure 3.16, two IGPs exist within the AS. All routers running RIP will learn about networks and subnets within their routing domain only. Routers in the IGRP domain will only learn routes from this domain. The router in the middle of the diagram is a border router, running more than one routing protocol (RIP and IGRP) and will therefore learn about routes from both routing domains.