BGP: Path Attributes

BGP PA define different characteristics about NLRIs associated with a PA. Each PA can be described as well known or optional.

Well known PAs are either one of the following:

• Mandatory: the PA MUST be in every update
• Discretionary: the PA is not require in every update

For example, the ATOMIC_AGGREGATE PA is a well-known discretionary PA, which means all routers must be able to understand this PA, but its not necessary in every update.

BGP classifies optional PAs into one of two categories:

• Transitive: The router should silently forward the PA to other routers without  needing to consider the meaning of the PA
• Nontransitive: The router should remove the PA so  that it is not propagated to any peers.