During BFD session establishment between devices, which state is not involved?

During the establishment of a Bidirectional Forwarding Detection (BFD) session between devices, the state progression follows a specific sequence to ensure that the link is operational and that both ends can communicate effectively. The states that are typically involved in this process are "down," "up," and "init."

The "down" state signifies that the BFD session has not yet been established, meaning no BFD control packets have been exchanged. The "init" state indicates that the BFD session has begun, where the device sends out BFD control packets to the other end. Once the devices recognize each other and establish a successful session, they transition to the "up" state, confirming that the BFD session is live and both devices are actively monitoring the link.

On the other hand, the "2-way" state is not a recognized state in the BFD session establishment process. This terminology is more commonly associated with protocols like OSPF, which describe neighbor relationships, rather than BFD. Therefore, "2-way" is not part of the state transitions in BFD and explains why it is the correct answer to the question.