In BGP, what is a path vector?

In BGP (Border Gateway Protocol), a path vector is fundamentally defined by a sequence of Autonomous System (AS) numbers. This sequence represents the path that routing information has taken through various autonomous systems, which are distinct networks managed by different organizations or entities.

When a BGP router advertises a route to a prefix, it also includes the AS path that the route has traversed. Each time a route propagates through an AS, it adds its own AS number to the path. This information serves multiple purposes: it helps routers determine the best path by evaluating the length of the AS path, prevents routing loops by allowing a router to reject paths that include its own AS number, and also provides visibility into the network topology.

The other options do not accurately describe the concept of a path vector in BGP. A list of IP addresses for routing pertains more to routing tables or forwarding rather than the specific mechanics of path advertisement in BGP. Methods of enhancing security in networks refer to various protocols and practices but do not directly encapsulate the path vector's role. An algorithm used for traffic engineering relates to the optimization of data flow within networks, which again diverges from the core definition of the BGP path vector that emphasizes AS sequences.