What marks the transition to a fully usable BGP session?

• A. An open message is sent.
• B. The router received agreement on the parameters for establishing a session.
• C. Peering is established and routing begins.
• D. The TCP handshake is initiated.

Answer: (C)

In BGP, the moment a session becomes fully usable is when peering is actually established and routing starts to flow between the peers. After the TCP connection is formed, BGP negotiates parameters via the OPEN exchange. Once both sides agree on those parameters and the session reaches the Established state, routers begin exchanging UPDATE messages and injecting routes into the local routing table. That is when the session can be used to carry real routing information.

The other steps are prerequisites: initiating the TCP handshake gets the connection going, and the OPEN/parameter agreement happens before you reach Established. But you don’t have usable routing until the peering is established and routing updates begin.