BGP: The Internet's Routing Protocol
Border Gateway Protocol (BGP) is how autonomous systems share routing information. Each AS announces the IP prefixes it controls to its neighbors, who propagate this information across the Internet. This creates a global routing table that every network uses to forward packets.
BGP operates on trust—networks accept route announcements from their peers. This design, while flexible, has led to notable incidents where misconfigured or malicious announcements have disrupted Internet traffic.
Major routing decisions consider AS path length (fewer hops preferred), local preferences, and commercial relationships (customer routes preferred over peer routes). Understanding AS paths helps diagnose why traffic takes unexpected routes.
Regional Internet Registries
Five Regional Internet Registries (RIRs) allocate IP addresses and ASNs globally:
ARIN - North America
RIPE NCC - Europe, Middle East, Central Asia
APNIC - Asia Pacific
LACNIC - Latin America, Caribbean
AFRINIC - Africa
These registries maintain WHOIS databases documenting who controls each IP range and ASN, essential for network operators, security researchers, and abuse handlers.
Peering and Transit
Peering: Two networks agree to exchange traffic directly, usually without payment. This happens at Internet Exchange Points (IXPs) or via private interconnections. Peering reduces latency and costs compared to routing through third parties.
Transit: A network pays another (typically a larger carrier) for access to the rest of the Internet. Transit providers agree to carry traffic to any destination, not just their own network.
The largest networks (Tier 1 providers) peer freely with each other, forming the Internet's core. Everyone else either pays for transit or peers strategically to reduce costs.
ASN Security and Monitoring
BGP Hijacking: Malicious or accidental announcements of IP prefixes belonging to other networks can redirect traffic. Route origin validation (RPKI) helps prevent this by cryptographically verifying announcement legitimacy.
Route Leaks: A network improperly announcing routes learned from one peer to another can cause traffic to flow through unintended paths, potentially overwhelming smaller networks.
Looking Glass Servers: Many networks provide public "looking glass" interfaces showing their BGP routing tables—valuable for diagnosing routing issues or verifying your announcements propagate correctly.