Which attributes are considered optional transitive?

• A. Aggregator and Community
• B. Local Preference and Atomic Aggregate
• C. AS Path and Origin
• D. Next Hop and Origin

Answer: (A)

In BGP, path attributes are categorized by whether they are well-known or optional, and by whether they are transitive or non-transitive. Optional transitive attributes are those that aren’t required to be understood by every router, but must be forwarded to the next hop along the path so that routers that do understand them can use them.

Aggregator is an optional transitive attribute. It carries information about the router that performed an aggregation and the origin of the route. If a neighbor doesn’t understand this attribute, it should still forward the update to the next router; if it does understand it, it can use the data for policy or troubleshooting. Community is also an optional transitive attribute. It provides a tagging mechanism to influence routing policies across multiple ASes. Networks commonly propagate communities even when downstream routers don’t interpret every value, preserving the information for those that do.

Other attributes listed aren’t optional transitive. Some are well-known and must be understood by all routers (like AS_PATH and Origin, and Next Hop). Local Preference is a well-known discretionary attribute that is intended for use within a single AS and is not forwarded across AS boundaries. Atomic Aggregate is an optional attribute that is not transitive, so it doesn’t fit the category of optional transitive.