What is the function of the BGP ‘AS Path’ attribute?

The function of the BGP ‘AS Path’ attribute is primarily to provide a loop prevention mechanism. In the context of BGP (Border Gateway Protocol), the AS Path is a list of the autonomous systems (AS) that a routing update has traversed. Each time a BGP update passes through a router, that router's AS number is appended to the AS Path.

When a BGP speaker receives a route advertisement, it checks the AS Path to see if its own AS number is already present in the path. If it finds its own AS number in the AS Path, it indicates that a routing loop could occur if the route was accepted. As a result, the speaker will reject the route update to avoid creating a loop in the routing table.

This mechanism ensures the integrity of the routing information disseminated within BGP, allowing networks to maintain stable and efficient routing tables while preventing loops that could lead to network failures or suboptimal routing paths. The presence of the AS Path attribute is thus critical in maintaining the reliability of BGP routing.