BGP
BGP is how autonomous systems announce address reachability, and the AS path shows the networks a route crosses.
Definitions only — a term explained here is not a service we sell.
BGP (Border Gateway Protocol) is how autonomous systems tell each other which address prefixes they can reach. Each announcement carries an AS path and the attributes best-path selection compares. An announcement states reachability, not performance.
BGP runs over TCP port 179. The paths shown here are illustrative examples, not copied from a live routing table.
The path an announcement travels
- AS64500 (documentation)
- AS64501 (documentation)
- AS64502 (documentation)
- Internet
Text description
Each network adds its own ASN to the front of the path before passing the route on. The last ASN is the origin; the first is the neighbor that handed you the route.
What to check
Ask which prefixes are announced, from which ASN, over how many upstreams, and whether the provider filters announcements with IRR objects or RPKI (RFC 6480). Adoption is uneven, so due diligence still matters.
When a route changes
During a maintenance window one provider can withdraw a prefix and another takes over; the AS path then grows from four hops to six. Review the path you expect against the one you see. The AS numbers guide covers the registry and peering practice.