What Is Interior Gateway Protocol and Exterior Gateway Protocol in Networking?

Routing protocols help routers determine where to send data so it can reach the correct destination. However, routing traffic within an organization's network is different from exchanging routes across the internet. This is where an Interior Gateway Protocol (IGP) and an Exterior Gateway Protocol (EGP) serve different purposes.

An interior gateway protocol manages routing within a single autonomous system, while an exterior gateway protocol handles routing between autonomous systems. Common IGP examples include OSPF, RIP, and EIGRP. Border Gateway Protocol (BGP) is the principal protocol used for exchanging routing information between autonomous systems on the internet. Understanding these protocols helps network engineers design reliable networks, troubleshoot connectivity issues, and manage how traffic reaches its destination.

What Is an Interior Gateway Protocol?

An Interior Gateway Protocol is used by routers to exchange routing information within the same autonomous system (AS). An autonomous system is a collection of IP networks, and routers managed under a common routing policy.

Organizations use IGPs to discover network paths, maintain routing tables, and respond to topology changes. When a link fails, an IGP can help routers identify alternative paths, depending on the protocol, available routes, and network configuration.

Common interior gateway protocols include the following.

• Open Shortest Path First (OSPF) is a link-state protocol. Routers exchange link-state information and calculate preferred paths using a shortest-path algorithm based on configured costs.

• Routing Information Protocol (RIP) is a distance-vector protocol that uses hop count as its routing metric. It supports a maximum of 15 hops to a destination, while 16 represents an unreachable destination.

• Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector protocol that uses the Diffusing Update Algorithm (DUAL) and a composite metric based on configured factors, including bandwidth and delay by default.

These protocols suit different network requirements. OSPF is commonly used in enterprise networks that need scalable internal routing, RIP may fit small or simple environments, and EIGRP can be useful where its capabilities and platform support align with the network design.

For example, if an organization has routers in its headquarters and multiple branches, an IGP can distribute routes between those locations so that employees and applications can communicate without manually configuring every destination on every router.

What Is an Exterior Gateway Protocol?

An Exterior Gateway Protocol enables routing information exchange between different autonomous systems. This is known as inter-domain routing and allows independently managed networks to communicate across the internet.

For example, an internet service provider (ISP) may exchange routes with another ISP so that customers on one network can reach destinations hosted on the other.

BGP is the principal protocol used for inter-domain routing on the public internet. It advertises IP prefixes and uses path attributes and routing policies to help determine which routes should be selected and shared with other networks.

Two BGP operating modes are particularly important.

• External BGP (eBGP) exchanges routing information between different autonomous systems.

• Internal BGP (iBGP) distributes BGP routing information between BGP speakers within the same autonomous system.

This distinction matters because BGP is classified as an exterior gateway routing protocol, even though iBGP operates within an autonomous system. An IGP may still be used alongside iBGP to provide internal reachability, including reachability to BGP next hops.

The older protocol specifically named Exterior Gateway Protocol also existed, but BGP has superseded it for modern internet inter-domain routing.

Interior Gateway Protocol vs Exterior Gateway Protocol

How IGP and EGP Work in a Real Network

Picture an enterprise with a core router, two distribution routers, and an edge router connected to an ISP. OSPF runs on all four, so each learns the internal subnets and the best internal path to each. That is the IGP at work.

The edge router also connects to the ISP, which has its own AS number. If the enterprise owns public address space and an AS number, the edge router can run eBGP with the ISP. It advertises the enterprise prefixes outward and receives either the ISP's routes or just a default route. The edge router then typically passes a default route into OSPF so internal routers know where to send outside traffic.

Not every enterprise needs BGP. A single-ISP site often uses a static default route instead. BGP becomes valuable with multiple providers, provider-independent address space, or specific policy needs. The two kinds of protocol are complementary, and one router often runs both.

Key Differences in Routing Decisions

IGPs choose routes using topology information and their own metric. OSPF prefers the lowest total cost, EIGRP the lowest composite metric, and RIP the lowest hop count. Equal-cost paths can often share traffic.

BGP does not simply pick the shortest physical path. Each route carries path attributes, and a best-path algorithm compares them in sequence. Exact steps vary slightly by vendor, but commonly include local preference (higher wins), AS path length, origin type, multi-exit discriminator (MED), preference for eBGP over iBGP, and the IGP metric to the next hop as a later tie-breaker. AS path length counts autonomous systems crossed, not distance or router hops. An operator can set local preference to favor a cheaper provider even when its AS path is longer. Policy often outweighs path length.

How to Choose the Right Routing Protocol

Engineers weigh a few factors:

• Network size and structure. Larger or hierarchical networks suit OSPF or IS-IS with areas, while RIP fits only tiny networks.

• Convergence requirements. OSPF and EIGRP generally recover faster than RIP.

• Operational complexity and skills. A protocol the team understands well is easier to run safely.

• Vendor environment. Multi-vendor networks often favor OSPF or IS-IS over EIGRP.

• Policy and connectivity. Multihoming, owned address space, or traffic control between providers points toward BGP at the edge.

A common design is one IGP inside the network and BGP only where it connects to other autonomous systems.

Final Takeaway

The difference between an interior gateway protocol and an exterior gateway protocol comes down to scope. IGPs such as OSPF, EIGRP, and RIP find paths inside an autonomous system, while BGP, the main exterior gateway protocol in use, exchanges policy-driven routes between them. Knowing where each belongs helps engineers design cleaner networks and troubleshoot routing problems faster.

Frequently Asked Questions

What is the main difference between IGP and EGP?

IGPs route within a single autonomous system, while EGPs route between autonomous systems. The first focuses on efficient internal paths, the second on controlled exchange between organizations.

Is BGP an interior or exterior gateway protocol?

BGP is classed as an exterior gateway protocol because it is designed for inter-domain routing. Its iBGP variant runs inside an AS but is still part of the same protocol.

Is OSPF an IGP or an EGP?

OSPF is an IGP. It distributes routes within one autonomous system and is not used between ASes.

What is an autonomous system in networking?

It is a group of IP networks and routers under one administrative control with a common routing policy, identified by a unique AS number.

Can a network use both IGP and BGP?

Yes. A router at the network edge often runs an IGP internally and BGP toward its ISP.

Which routing protocols are examples of IGPs?

OSPF, IS-IS, EIGRP, and RIP are the common examples.

How do IGPs and BGP differ in route selection?

IGPs mainly compare metrics such as cost or hop count. BGP evaluates path attributes and policy settings in a defined order.