Junos Enterprise Routing, 2nd Edition

IGP Overview

As its name would imply, the role of an IGP is to provide routing connectivity within or interior to a given routing domain (RD). An RD is defined as a set of routers under common administrative control that share a common routing protocol. An enterprise network, which can also be considered an autonomous system (AS), may consist of multiple RDs, which may result from the (historic) need for multiple routed protocols, scaling limitations, acquisitions and mergers, or even a simple lack of coordination among organizations making up the enterprise. Route redistribution, the act of exchanging routing information among distinct routing protocols, is often performed to tie these RDs together when connectivity is desired.

IGP functions to advertise and learn network prefixes (routes) from neighboring routers to build a route table that ultimately contains entries for all sources advertising reachability for a given prefix. A route selection algorithm is executed to select the best (i.e., the shortest) path between the local router and each destination, and the next hop associated with that path is pushed into the forwarding table to affect the forwarding of packets that longest-match against that route prefix. The IGP wants to provide full connectivity among the routers making up an RD. Generally speaking, IGPs function to promote, not limit, connectivity, which is why we do not see IGPs used between ASs—they lack the administrative controls needed to limit connectivity based ...