BFD session modes

BFD sessions use echo packets and control packets.

Echo packets are encapsulated into UDP packets with port number 3785.

The local end of the link sends echo packets to establish BFD sessions and monitor link status. The peer end does not establish BFD sessions and only forwards the packets back to the originating end. If the local end does not receive echo packets from the peer end within the detection time, it considers the session to be down.

In echo packet mode, BFD supports multihop detection only for MPLS TE tunnel scenarios. Both BFD sessions for single-hop detection and BFD sessions for multihop detection are independent of the operating mode.

Control packets are encapsulated into UDP packets with port number 3784 for single-hop detection or port number 4784 for multihop detection.

Both ends of the link exchange BFD control packets to monitor link status.

Before a BFD session is established, BFD has two operating modes—active and passive.

• Active mode—BFD actively sends BFD control packets regardless of whether any BFD control packet is received from the peer.

• Passive mode—BFD does not send control packets until a BFD control packet is received from the peer.

At least one end must operate in active mode for a BFD session to be established.

After a BFD session is established, the two ends can operate in the following BFD operating modes:

• Asynchronous mode—The device periodically sends BFD control packets. The device considers that the session is down if it does not receive any BFD control packets within a specific interval.

• Demand mode—The device periodically sends BFD control packets. If the peer end is operating in Asynchronous mode (default), the peer end stops sending BFD control packets. If the peer end is operating in Demand mode, both ends stop sending BFD control packets. When the connectivity to another system needs to be verified explicitly, a system sends several BFD control packets with the Poll (P) bit set at the negotiated transmit interval. If no response is received within the detection interval, the session is considered down. If the connectivity is found to be up, no more BFD control packets are sent until the next command is issued.