USG A and USG B are configured with a static BFD session.

Note: 1. USG A and USG B each start the BFD state machine. The initial state is Down and the BFD packet is Down. For a static BFD session, the value of the Your Discriminator is specified by the user. For the dynamic BFD session, the value of the Your Discriminator is 0. 2. After receiving the BFD packet whose status is Down, the USG B switches to Init. And send a BFD packet with the status of Init. 3. If the local BFD state of USG B is Init, the packets of the received state are Down. 4. The BFD state of USG A is the same as that of USG B. 5. After receiving the BFD packet in the Init state, the local state is switched to Up. 6. The BFD status of USG A changes with USG B. 7. After the state transition of "DOWN-->INIT" occurs on USG A and USG B, a timeout timer is started. If the BFD packet is in the Init or Up state, the local state is automatically switched back to Down.