Intra- and Inter-Domain Routing

Internet routing is organized into two tiers: interior protocols such as OSPF and RIP that route within a single administrative domain, and the exterior Border Gateway Protocol that routes between the autonomous systems making up the global Internet.

Definition

Intra-domain routing is the determination of paths within a single autonomous system using an interior gateway protocol, while inter-domain routing is the determination of paths between autonomous systems using the exterior Border Gateway Protocol, which exchanges policy-controlled reachability information.

Scope

This topic covers the protocols that realize routing in the real Internet. Within an autonomous system (intra-domain) it treats interior gateway protocols, principally the link-state protocol OSPF and the distance-vector protocol RIP. Between autonomous systems (inter-domain) it treats the path-vector Border Gateway Protocol, including how it propagates reachability, supports policy, and enables business relationships such as transit and peering. It builds on the abstract routing algorithms and excludes their underlying graph theory.

Core questions

• Why is Internet routing split into intra-domain and inter-domain tiers?
• How do interior protocols such as OSPF and RIP differ in their routing approach?
• How does BGP propagate reachability information between autonomous systems?
• How does BGP support policy and business relationships rather than pure shortest paths?
• What stability and security challenges arise in inter-domain routing?

Key concepts

• autonomous system (AS)
• interior gateway protocols
• OSPF (link-state)
• RIP (distance-vector)
• Border Gateway Protocol (BGP)
• path-vector routing
• routing policy
• transit and peering
• BGP route propagation

Key theories

Two-tier routing hierarchy

Routing is divided into interior gateway protocols that optimize paths within an autonomous system and an exterior protocol that connects autonomous systems, decoupling internal optimization from global reachability and respecting administrative boundaries.

Path-vector, policy-based inter-domain routing

BGP advertises the full autonomous-system path to each destination prefix, which lets networks detect loops and apply local policy preferences based on business relationships rather than choosing strictly shortest paths.

Link-state interior routing

OSPF distributes link-state advertisements within a domain so that each router computes shortest paths via Dijkstra's algorithm, supporting fast convergence, areas for scalability, and multiple cost metrics.

Clinical relevance

These protocols run the operational Internet: OSPF and similar interior protocols keep large enterprise and provider networks converged after failures, while BGP knits tens of thousands of autonomous systems into a single reachable Internet. BGP's policy mechanisms encode the economics of the Internet, and BGP misconfigurations and hijacks are a recurring source of large-scale outages and security incidents.

History

Early interior routing used the distance-vector RIP, later supplemented by the link-state OSPF (RFC 2328) for larger networks. As the Internet became a network of independently run networks, the Exterior Gateway Protocol and then BGP (current version BGP-4, RFC 4271) emerged to handle policy-driven routing between autonomous systems, and BGP has remained the inter-domain routing protocol of the Internet ever since.

Debates

Securing BGP without breaking deployability

BGP trusts route advertisements by default, enabling route hijacks and leaks; proposed defenses such as RPKI origin validation and path validation improve security but face incremental-deployment and operational hurdles, so adoption has been gradual and partial.

Key figures

• Yakov Rekhter
• John Moy
• Jennifer Rexford

Seminal works

• rfc4271
• rfc2328
• kurose2021

Frequently asked questions

What is an autonomous system?

An autonomous system is a network or group of networks under a single administrative control, such as an Internet service provider or a large enterprise, that presents a unified routing policy to the rest of the Internet. Each is identified by an AS number, and BGP routes traffic between these systems.

Why doesn't BGP just pick the shortest path?

Inter-domain routing reflects business and policy choices, not only path length. A network may prefer to route through a partner it peers with for free over a shorter path through a provider it pays, or avoid certain networks entirely. BGP lets each autonomous system apply such policies when selecting and advertising routes.