BGP attributes are a confusing array of information carried in a BGP update capable of indicating anything from path preference to various additional pieces of information about a route, either within an autonomous system or outside an autonomous system. There are four basic types of attributes:
- Well known mandatory attributes; these attributes must be recognized by all BGP speakers, and must be included in all update messages. Almost all of the attributes impacting the path decision process ( Origin Code , AS Path , Next Hop).
- Well known discretionary attributes; these attributes must be recognized by all BGP speakers, and may be carried in updates, but are not required in every update.
- Optional transitive attributes; these attributes may be recognized by some BGP speakers, but not all. They should be preserved and advertised to all peers whether or not they are recognized.
- Optional non-transitive attributes; these attributes may be recognized by some BGP speakers, but not all. If an update containing an optional transitive attribute is received, the update should be advertised to peers without the unrecognized attributes.
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