Interior Gateway Protocol (IGP): routing inside an autonomous system

Key characteristics

IGPs are designed to meet constraints and goals that differ from protocols used between autonomous systems. Typical characteristics include fast convergence after topology changes, scalability appropriate to the size of the AS, and metrics that reflect internal link qualities rather than business policies. Common features and design choices are:

• Routing algorithm families: distance-vector, link-state, and hybrid approaches.
• Metrics: hop count, bandwidth-based cost, delay, or composite metrics used to select best paths.
• Hierarchical design: mechanisms such as areas or levels reduce routing table size and limit update scope.
• Administrative and operational settings: timers, route summarization, authentication, and administrative distance.

Major protocols and examples

• RIP (Routing Information Protocol): a simple distance-vector protocol using hop count as its metric; it is limited in scale by a maximum hop count.
• OSPF (Open Shortest Path First): a widely used link-state protocol that supports areas, authentication, and cost metrics often tied to bandwidth.
• IS-IS (Intermediate System to Intermediate System): another link-state protocol with flexible design and common use in large service-provider networks.
• EIGRP (Enhanced Interior Gateway Routing Protocol): originally developed by a single vendor, it is a hybrid protocol combining features of distance-vector and link-state approaches.

History and development

IGPs evolved as networks grew beyond simple topologies. Early protocols emphasized simplicity and low overhead; as networks expanded, link-state protocols were developed to improve convergence and provide richer metrics. Over time, hybrid techniques and hierarchical structures were introduced to balance performance, scalability, and operational complexity.

Uses, benefits and considerations

IGPs are the default choice for routing internal traffic within an AS. They are suited to enterprise campuses, data centers, and internal segments of service providers. Network designers choose an IGP based on size, administrative boundaries, convergence needs, and available equipment. Important trade-offs include protocol complexity versus ease of management, speed of convergence versus resource consumption, and how well the metric reflects application requirements.

Distinctions and notable facts

IGPs contrast with exterior gateway protocols (EGPs) that exchange routes between autonomous systems; the best-known EGP is BGP. IGPs focus on topological correctness and performance inside the AS rather than on interdomain policy. For more on topology challenges and design patterns see network topology references.