Path Vector protocols are similar and practically the same as distance vector. routers exchange messages about known networks and their distances, however in addition each network is accompanied by a list of path elements describing the path toward each network being advertised. The path element can be represented by a number of different identifiers, BGP is the most widely used path vector protocol, and it uses the list of autonomous systems traffic must traverse to get to the destination network as its path element.
While this can be used to determine the best path to a destination, its primary function is for loop avoidance. Routers will not accept route advertisements to networks whose path description already includes the identifier the router would include into it’s advertisements itself.