BGP attribute recording each ASN a route has passed through. Routes with shorter paths usually win, so adding extra ASNs makes one look less attractive.
Also called autonomous system path.
Read more: Microsoft Learn
In the Ultra Transcenders books
Each book explains AS path in context, with comparison tables and the common traps.
Terms in this definition
- BGP
Dynamic routing protocol used with both ExpressRoute and VPN connections. On ExpressRoute private peering it is the only way to exchange routes, including a 0.0.0.0/0 default route for forced tunnelling.
- ASN
Short for autonomous system number, which identifies a BGP routing domain. Azure VPN gateways use 65515 by default, a reserved value on-premises peers must avoid; Defender EASM also discovers ASNs as an internet asset type.
Related terms
- AS Path (hub routing preference)
Routing preference for a Virtual WAN hub under which the shortest BGP AS path wins regardless of where the route came from. When local routes tie, ExpressRoute is chosen over site-to-site VPN.
- AS-path prepending
Making a BGP route less attractive by repeating ASNs to lengthen its AS path, steering traffic towards a different site or path.
- Hub routing preference
A virtual hub option, set to ExpressRoute by default or to VPN or AS Path, that decides between on-premises routes with the same prefix once longest prefix match and the static-over-BGP rule have been applied.