BFD session modes and operating modes

BFD sessions use the following types of packets:

Echo packet mode

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.

In echo packet mode, BFD supports only single-hop detection and the BFD session is independent of the operating mode.

Control packet mode

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.

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:

In addition, both ends of the link can exchange BFD control packets to establish and maintain BFD sessions, and one end of the link sends echo packets to monitor link status.