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Types of Routing Protocols: IGPs and EGPs
Learn how routing protocols are classified as IGPs and EGPs, how autonomous systems work, and where RIP, OSPF, IS-IS, EIGRP, and BGP are used.
Routers need a way to learn which networks are reachable and which paths should be used to reach them. A routing protocol is a protocol that routers use to exchange reachability and path information so routes can be learned dynamically.
Routing protocols are part of dynamic routing. A dynamic route is learned through a routing protocol. By contrast, a static route is entered manually by an administrator. Static routes can be useful for simple or highly predictable paths, while dynamic routing allows routers to respond more easily to network changes.
What Are the Two Main Types of Routing Protocols?
Routing protocols are commonly classified according to whether they operate inside one administrative network or exchange routes between separately administered networks:
- Interior Gateway Protocols (IGPs) operate within a single autonomous system.
- Exterior Gateway Protocols (EGPs) exchange routing information between autonomous systems.
The terms describe categories, not usually the names of individual protocols. OSPF, RIP, IS-IS, and EIGRP are examples of IGPs. BGP is the widely deployed example of an EGP.
Understanding Autonomous Systems
An autonomous system (AS) is a network, or a collection of interconnected networks, operated under one administrative authority. An enterprise, university, cloud provider, or Internet service provider can operate an AS.
An AS does not have to be one physical network. For example, a company can place its headquarters, branch offices, data centers, and private WAN links in the same AS when they are managed as one routing domain.
The AS boundary separates internal routing from inter-domain routing. Routers inside the boundary use an IGP to exchange internal reachability information. Edge routers can use BGP to exchange routes with a different organization or service provider.
Interior Gateway Protocols (IGPs)
An Interior Gateway Protocol (IGP) is used for routing within one autonomous system. It distributes internal network reachability among routers that belong to the same administrative domain.
For example, a company with headquarters, branches, and data centers can use an IGP to advertise its internal subnets. When a link fails or a new subnet is added, the IGP can help routers learn an updated path without requiring every route to be entered manually.
Common IGP Examples
- RIP — Routing Information Protocol, a distance-vector IGP.
- OSPF — Open Shortest Path First, a link-state IGP.
- IS-IS — Intermediate System to Intermediate System, a link-state IGP.
- EIGRP — Enhanced Interior Gateway Routing Protocol, an IGP commonly encountered in enterprise environments.
IGP selection depends on the needs and design of the internal network. Factors can include network size, convergence requirements, operational familiarity, scalability, and vendor support.
Distance-Vector and Link-State IGP Families
IGPs can also be discussed according to how they learn and calculate routes. Two important families are distance-vector and link-state routing.
Distance-Vector Routing
In distance-vector routing, routers learn route information from neighboring routers. They use advertised routing data and metrics to select paths. A metric is a value used to compare possible routes, such as hop count or another measure of path cost.
RIP is the classic example of a distance-vector IGP. A router does not need a complete map of every link in the network; instead, it receives route information from its neighbors and uses that information to choose a path.
Link-State Routing
In link-state routing, routers exchange information about the state of their links. Each router uses that information to build a view of the network topology, sometimes called a link-state database. It then calculates best paths across that topology.
OSPF and IS-IS are link-state IGPs. This approach gives routers a more complete view of the internal network and is commonly used in larger or more structured environments.
Where EIGRP Fits
EIGRP is a commonly encountered IGP. Its route calculation behavior is more sophisticated than the basic distance-vector model, so it should not be casually treated as identical to simple distance-vector protocols. For introductory classification, the important point is that EIGRP is an IGP used inside an autonomous system.
Exterior Gateway Protocols (EGPs)
An Exterior Gateway Protocol (EGP) exchanges routing information between separate autonomous systems. This is called inter-domain routing and is closely associated with Internet-scale routing and connections between independent organizations.
The widely deployed exterior routing protocol is BGP, or Border Gateway Protocol. BGP exchanges routes between different autonomous systems, including networks connected to the public Internet. For example, an enterprise can use BGP to exchange routes with its Internet service provider.
BGP is designed for policy-based inter-domain routing. Separate organizations may make routing decisions based not only on the shortest technical path, but also on business relationships, traffic policies, redundancy, and security requirements.
IGP Versus EGP Comparison
IGPs and BGP can coexist in the same organization. A service provider might use an IGP for reachability between its internal routers while using BGP at its external boundaries to exchange routes with customers, peers, and other providers.
Common Routing Protocol Examples
Practical Examples
One Company With Several Locations
A company operates headquarters, branch offices, and data-center networks under one administrative team. Because the locations belong to one AS, the company can use an IGP such as OSPF to exchange internal routes. The routers use the learned information to forward traffic between company subnets.
Two Independent Organizations
An enterprise connects to an Internet service provider. The enterprise and the provider operate separate autonomous systems. BGP is used across the connection to exchange inter-domain routing information, while each organization can use its own IGP internally.
Internal and External Routing Together
A service provider can use an IGP to ensure that its internal routers can reach one another. At the same time, BGP can advertise customer and Internet routes to neighboring autonomous systems. The IGP provides internal transport, while BGP handles external route exchange.
Common Misconceptions
IGP and EGP Are Not Usually Individual Protocol Names
IGP and EGP are categories. RIP, OSPF, IS-IS, and EIGRP are IGP examples. BGP is the primary modern EGP example.
BGP Is Not the Normal Internal Protocol for Every Enterprise Router
BGP is primarily used between autonomous systems. An IGP commonly carries internal routing information within an enterprise or service provider. Some organizations may use BGP internally for specific designs, but that does not change its role as an inter-domain protocol.
An AS Is Not Necessarily One Physical Network
An autonomous system can include many interconnected networks, sites, and routers. The defining characteristic is common administrative control, not physical size or location.
Administrative Distance Is Not a Protocol Type
Administrative distance is a route-source preference value. When a router learns a route to the same destination from different sources, administrative distance can help the router prefer one source. It is not an IGP, EGP, distance-vector protocol, or link-state protocol.
How This Fits Into Dynamic Routing
Dynamic routing is the broader practice of learning and maintaining routes through routing protocols. IGPs and EGPs describe the scope in which those protocols exchange information.
Routing metrics are another separate concept. A metric helps a protocol compare paths. Administrative distance is also separate: it helps a router compare routes learned from different sources. Do not confuse a protocol's routing family, its metric, and administrative distance.
Exam-Ready Summary
- An autonomous system is a network or group of networks managed by one administrative authority.
- An IGP routes within one AS.
- An EGP exchanges routes between ASes.
- RIP, OSPF, IS-IS, and EIGRP are common IGP examples.
- RIP is associated with distance-vector routing.
- OSPF and IS-IS are link-state protocols.
- EIGRP is an IGP and should not be oversimplified as a basic distance-vector protocol.
- BGP is the widely deployed protocol for routing between autonomous systems.
- An organization can use an IGP internally and BGP at its external boundaries.
- Administrative distance is a route-selection preference, not a routing-protocol category.
For a related overview, see Types of Routing Protocols.