Description: The state of the running BFD session.

There are three states through which a session normally proceeds, two for establishing a session (Init and Up) and one for tearing 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.) A fourth state (AdminDown) exists so that a session can be administratively put down indefinitely.

The Down state means that the session is down (or has just been created.) A session remains in Down state until the remote system indicates that it agrees that the session is down by sending a BFD Control packet with the State field set to anything other than Up. If that packet signals Down state, the session advances to Init state; if that packet signals Init state, the session advances to Up state.

Init state means that the remote system is 0communicating, and the local system desires to bring the session up, but the remote system does not yet realize it. A session will remain in Init state until either a BFD Control Packet is received that is signaling Init or Up state (in which case the session advances to Up state) or until the detection time expires, meaning that communication with the remote system has been lost (in which case the session advances to Down state.)

Up state means that the BFD session has successfully been established, and implies that connectivity between the systems is working. The session will remain in the Up state until either connectivity fails, or the session is taken down administratively. If either the remote system signals Down state, or the detection time expires, the session advances to Down state.

AdminDown state means that the session is being held administratively down. This causes the remote system to enter Down state, and remain there until the local system exits AdminDown state.