interAS

InterAS, or inter-Autonomous System routing, refers to the set of mechanisms by which different Autonomous Systems (ASes) on the Internet exchange routing information and reachability. Inter-AS routing is distinct from intra-AS routing, which occurs inside a single AS. The Border Gateway Protocol (BGP) is the principal protocol used to exchange routing information between ASes, with sessions typically established between neighboring networks.

In inter-AS routing, each AS uses eBGP (external BGP) sessions with directly connected neighbor ASes to advertise reachability to prefixes; within an AS, iBGP sessions carry routing information among internal routers. BGP conveys not only prefixes but attributes such as AS_PATH, NEXT_HOP, and policy-related attributes that influence path selection. Operators deploy peering arrangements at Internet Exchange Points and via transit agreements with tier-1 or tier-2 providers to reach destinations.

Routing decisions across AS boundaries rely on operator policies. Local preference, AS_PATH length, MED, and other attributes influence which inter-domain path is chosen. To scale, networks often use route reflectors or confederations for iBGP, and employ prefix filtering, route-servers at IXPs, and strict peering policies to control advertisement and acceptance of routes.

Inter-AS routing presents security and reliability challenges. BGP itself lacks inherent validation, making routes vulnerable to misconfiguration or hijacking. Mitigations include RPKI-based origin validation, BGPsec, prefix filtering, and anomalies such as unstable withdrawals. Operators also implement redundancy through multiple interconnections and multihoming.

Given its role in global connectivity, inter-AS routing shapes Internet reachability, performance, and resilience. The ecosystem relies on a combination of peering, transit, IXPs, and policy coordination among many independent networks.