IGPS

Interior Gateway Protocols (IGPs) are routing protocols used by routers within a single administrative domain, or autonomous system, to exchange routing information and determine paths to destinations inside that domain. IGPs operate alongside Exterior Gateway Protocols (EGPs) such as BGP, which are used to exchange routing information between autonomous systems. IGPs differ from EGPs in scope, design, and metrics, and are designed to support fast convergence and efficient use of network resources inside a single organization.

Common IGPs include RIP, OSPF, IS-IS, and EIGRP. They are broadly categorized into distance-vector (RIP, EIGRP) and link-state (OSPF, IS-IS) protocols. RIP uses hop count with a maximum of 15 hops; OSPF and IS-IS build a database of the network topology and compute shortest paths with the SPF algorithm; EIGRP uses a diffusing update algorithm with composite metrics. OSPF organizes networks into areas with a backbone area (0) to improve scalability; IS-IS uses a two-level hierarchy.

Deployment choices depend on network size, vendor support, and required features. RIP is simple but limited in scale, while OSPF and IS-IS are favored in medium to large networks due to scalability and fast convergence. EIGRP provides fast convergence and flexible metrics in Cisco-centric environments. IGPs typically support fast failover, route redistribution, and summarization, and are tuned with timers, hello intervals, and route filtering.

In practice, an enterprise may run one dominant IGP across its campus and data centers, with BGP as a boundary router to external networks. Network engineers monitor LSDB convergence, SPF computations, and stability to ensure reliable routing.