Path Attributes
Path attributes are included in the UPDATE messages to help BGP systems compare routes and decide which route to use when given multiple options. Path attributes fall into four categories:
Well-known attributes are supported by all implementations of BGP. Mandatory ones must be included in all UPDATE messages, but discretionary ones may not be. When a route is passed along to neighbors, all well-known attributes must be passed along with it.
Optional attributes are not necessarily supported by all BGP implementations. Transitive one should be passed along, even if not recognized by the system. Non-transitive attributes which are not recognized are discarded. All optional attributes may be updated by ASs along the path.
Path attributes include