7.2.1 INTRODUCTION

An Interior Gateway Protocol (IGP) is used to distribute routing information between the various routers in a particular AS. Independent of the algorithm used to implement a particular IGP, it should perform the following functions:

1. Respond quickly to changes in the internal topology of an AS

2. Provide a mechanism such that circuit flapping does not cause continuous routing updates

3. Provide quick convergence to loop-free routing

4. Utilize minimal bandwidth

5. Provide equal cost routes to enable load-splitting

6. Provide a means for authentication of routing updates

Current IGPs used in the internet today are characterized as either being based on a distance-vector or a link-state algorithm.

Several IGPs are detailed in this section, including those most commonly used and some recently developed protocols that may be widely used in the future. Numerous other protocols intended for use in intra-AS routing exist in the Internet community.

A router that implements any routing protocol (other than static routes) MUST IMPLEMENT OSPF (see Section [7.2.2]). A router MAY implement additional IGPs.