Path vector routing is discussed in

Path vector routing is discussed in RFC 1322; the following paragraphs are based on the RFC.The path vector routing algorithm is somewhat similar to the distance vector algorithm in the sense that each border router advertises the destinations it can reach to its neighboring router. However, instead of advertising networks in terms of a destination and the distance to that destination, networks are advertised as destination addresses and path descriptions to reach those destinations.

A route is defined as a pairing between a destination and the attributes of the path to that destination, thus the name, path vector routing, where the routers receive a vector that contains paths to a set of destinations.
The path, expressed in terms of the domains (or confederations) traversed so far, is carried in a special path attribute that records the sequence of routing domains through which the reachability information has passed. The path represented by the smallest number of domains becomes the preferred path to reach the destination.
The main advantage of a path vector protocol is its flexibility. There are several other advantages regarding using a path vector protocol:
- The computational complexity is smaller than that of the link state protocol. The path vector computation consists of evaluating a newly arrived route and comparing it with the existing one, while conventional link state computation requires execution of an SPF algorithm.
- Path vector routing does not require all routing domains to have homogeneous policies for route selection; route selection policies used by one routing domain are not necessarily known to other routing domains. The support for heterogeneous route selection policies has serious implications for the computational complexity. The path vector protocol allows each domain to make its route selection autonomously, based only on local policies. However, path vector routing can accommodate heterogeneous

route selection with little additional cost.
_ Only the domains whose routes are affected by the changes have to recompute.
_ Suppression of routing loops is implemented through the path attribute, in contrast to link state and distance vector, which use a globally-defined monotonically thereby increasing metric for route selection. Therefore, different confederation definitions are accommodated because looping is avoided by the use of full path information.
_ Route computation precedes routing information dissemination. Therefore, only routing information associated with the routes selected by a domain is distributed to adjacent domains.
_ Path vector routing has the ability to selectively hide information. However, there are disadvantages to this approach, including:
_ Topology changes only result in the recomputation of routes affected by these changes, which is more efficient than complete recomputation. However, because of the inclusion of full path information with each distance vector, the effect of a topology change can propagate farther than in traditional distance
vector algorithms.

_ Unless the network topology is fully meshed or is able to appear so, routing loops can become an issue.
BGP is a popular example of a path vector routing protocol