IGP

An interior gateway protocol (IGP) is a routing protocol used to exchange routing information within a single autonomous system or administrative domain. IGPs enable routers inside the same network to determine optimal paths to internal destinations. They differ from exterior gateway protocols (EGPs), which govern routing between autonomous systems; the most widely used EGP is the Border Gateway Protocol (BGP).

IGPs are typically categorized into two main families: distance-vector and link-state. Distance-vector protocols, such as RIP and EIGRP, determine routes based on hop counts or other simple metrics and gradually propagate routing information. Link-state protocols, such as OSPF and IS-IS, build a complete view of the network topology and compute shortest paths using a graph-based algorithm. Link-state protocols generally offer faster convergence and greater scalability in larger networks, while distance-vector protocols tend to be simpler to configure.

Common IGPs include:

- OSPF (Open Shortest Path First), a widely deployed link-state protocol that supports hierarchical network design with areas and uses cost based on link bandwidth.

- IS-IS (Intermediate System to Intermediate System), another link-state protocol used in large enterprises and service provider networks, known for strong scalability and flexible topology support.

- RIP (Routing Information Protocol), a basic distance-vector protocol with a small hop-limit, suitable for simple networks but limited in scale.

- EIGRP (Enhanced Interior Gateway Routing Protocol), a Cisco-proprietary protocol that blends distance-vector and capabilities typically associated with link-state approaches.

In practice, network designers choose IGPs based on size, performance, and administrative requirements, often deploying OSPF or IS-IS for core routing and using IGP features such as route summarization, area design, and authentication to control routing behavior.