A session normally proceeds through three states; two states are used to establish a session (Init and Up) and one state is used to tear down a session (Down). This allows a three-way handshake for both session establishment and session teardown, assuring that both systems are aware of all session state changes. There is a fourth state (AdminDown) that you can use to administratively put a session down indefinitely.
Down state: Indicates the session is down or has just been created. The session will remain in Down state until the remote system sends a BFD control packet indicating anything other than Up state. If the control packet signals Down state, the session advances to Init state. If the control packet signals Init state, the session advances to Up state.
Init state: In this state, the host system establishes communications with the remote system and sends a request to move the session to the Up state, but the remote system has not yet recognized the request. A session remains in Init state until it receives a BFD control packet signaling Init or Up state, or until the connectivity timer expires, indicating communication with the remote system is lost.
Up state: Indicates the BFD session is established and connectivity is working. A session remains in Up state until connectivity fails or until the session is taken down administratively.
AdminDown state: Indicates the BFD session is being held down administratively. This causes the remote system to enter Down state and remain there until the local system exits AdminDown state.