edVirtus Reference

What Are Interior and Exterior Gateway Protocols?

How Do Gateway Protocols Exchange Routing Information Within and Between Networks?

Gateway protocols are communication protocols that enable routers to exchange routing information and determine the paths that packets should follow through interconnected networks. They automatically distribute information about network topology, allowing routers to adapt to changing network conditions without requiring manual configuration. Gateway protocols are divided into two broad categories: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).

The basic principle is straightforward. Routers continually exchange information describing the networks they can reach and the quality of the available paths. Each router uses this information to build and maintain a routing table that identifies the preferred route to every known destination. When network links fail or new networks are added, the routing protocol automatically updates the routing tables so that communication can continue with minimal disruption.

A useful analogy is a road information service. Local traffic authorities manage roads within their own city, while national highway authorities coordinate routes between cities and states. Drivers rely on information from both organisations to determine the best route to their destination. Similarly, IGPs manage routing within an organisation, while EGPs coordinate routing between independent organisations.

An Interior Gateway Protocol (IGP) operates within a single Autonomous System (AS). Because all routers belong to the same organisation and share a common administrative policy, IGPs can optimise routes based on technical criteria such as link bandwidth, delay, or hop count. Common IGPs include Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), Routing Information Protocol (RIP), and Enhanced Interior Gateway Routing Protocol (EIGRP). These protocols are widely used within enterprise networks, university campuses, and Internet service provider networks.

An Exterior Gateway Protocol (EGP) operates between Autonomous Systems. The only EGP in widespread use today is the Border Gateway Protocol (BGP). Rather than simply selecting the technically shortest path, BGP allows organisations to apply routing policies based on commercial agreements, security requirements, traffic engineering, and administrative preferences. This policy-based routing is essential for the operation of the global Internet, where thousands of independently managed Autonomous Systems must cooperate while maintaining their own routing policies.

It is important to distinguish IGPs from EGPs. IGPs exchange routing information between routers under the control of a single organisation and generally seek the most efficient technical route. EGPs exchange routing information between different organisations and emphasise routing policy, scalability, and administrative independence. Likewise, gateway protocols differ from static routing, in which routing tables are entered manually and do not adapt automatically to network changes.

Today, Interior and Exterior Gateway Protocols provide the routing intelligence that enables modern IP networks to function. IGPs ensure efficient communication within organisations, while BGP interconnects more than one hundred thousand Autonomous Systems to form the global Internet. Together, these protocols allow data to be routed reliably and efficiently across networks ranging from small enterprise LANs to worldwide communication infrastructures.