AdministrativelyDownThe local system does not participate in a particular session.Down The remote system is not sending control packets or at least not within the detection time for a particularsession.Init The local system is communicating.Up Both systems are exchanging control packets.The session is declared down if:• A control packet is not received within the detection time.• Sufficient echo packets are lost.• Demand mode is active and a control packet is not received in response to a poll packet.BFD Three-Way HandshakeA three-way handshake must take place between the systems that participate in the BFD session.The handshake shown in the following illustration assumes that there is one active and one passive system, and that this session isthe first session established on this link. The default session state on both ports is Down.1. The active system sends a steady stream of control packets that indicates that its session state is Down, until the passivesystem responds. These packets are sent at the desired transmit interval of the Active system. The Your Discriminator field isset to zero.2. When the passive system receives any of these control packets, it changes its session state to Init and sends a response thatindicates its state change. The response includes its session ID in the My Discriminator field and the session ID of the remotesystem in the Your Discriminator field.3. The active system receives the response from the passive system and changes its session state to Up. It then sends a controlpacket indicating this state change. This is the third and final part of the handshake. Now the discriminator values have beenexchanged and the transmit intervals have been negotiated.4. The passive system receives the control packet and changes its state to Up. Both systems agree that a session has beenestablished. However, because both members must send a control packet — that requires a response — anytime there is astate change or change in a session parameter, the passive system sends a final response indicating the state change. Afterthis, periodic control packets are exchanged.Bidirectional Forwarding Detection (BFD) 123
