Border Gateway Protocol (BGP)
Border Gateway Protocol · Also known as: exterior gateway protocol, inter-domain routing
BGP is the de facto standard routing protocol for interconnecting autonomous systems (ASs) on the Internet. Since its introduction in 1989, BGP has scaled the Internet to millions of routers and trillions of destinations. BGP is path-vector-based, using a flexible policy system to control route propagation and selection. While BGP convergence can be slow and policies complex, it remains the only viable protocol for Internet-scale inter-domain routing.
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When to use it
BGP is mandatory for any AS connecting to the Internet. Use BGP for inter-domain routing and multi-homing (multiple ISP connections). Avoid BGP for intra-domain routing (use OSPF/IS-IS); BGP convergence is slow. Policy configuration in BGP is powerful but complex; experienced operators are needed.
Strengths & limitations
- Internet-scale: proven to scale to millions of routes and routers
- Flexible policy system: fine-grained control over route propagation and selection
- Loop prevention: AS-path prevents routing loops between domains
- Path hiding: routers don't know all paths, only best path
- Stability: loop-free and convergent under any configuration
- Convergence latency: re-convergence after topology changes can take minutes
- Policy complexity: misconfigured policies cause route filtering and suboptimal paths
- BGP blackholing: traffic can be attracted by routes with long AS-paths; ASPA/ROV mitigation incomplete
- Route hijacking: prefix hijacking is possible; RPKI/ROA are recent but not universally deployed
- State explosion: millions of routes consume memory and CPU
Frequently asked
What is the difference between iBGP and eBGP?
eBGP connects routers in different ASs; advertisements include true AS-path. iBGP connects routers within same AS; AS-path unchanged internally. iBGP requires full mesh or route reflectors to scale.
How does BGP prevent routing loops?
AS-path lists all ASs a route traversed; a router rejects routes with its own AS in the path. This guarantees loop-free paths between ASs.
What is route filtering and when is it used?
Route filters (import/export policies) allow operators to accept only certain routes from neighbors or advertise only certain routes to them. Essential for traffic engineering and preventing route hijacks.
What is RPKI and why is it important?
RPKI (Resource Public Key Infrastructure) cryptographically binds IP prefixes to ASNs. Routers using RPKI validation (ROV) reject route advertisements claiming prefixes they don't own, preventing hijacking.
Sources
How to cite this page
ScholarGate. (2026, June 3). Border Gateway Protocol. ScholarGate. https://scholargate.app/en/telecommunications/bgp
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