Dell S4048T Configuration Manual page 181

1 The active system sends a steady stream of control packets that indicates that its session state is Down,
until the passive system responds. These packets are sent at the desired transmit interval of the Active
system. The Your Discriminator field is set to zero.
2 When the passive system receives any of these control packets, it changes its session state to Init and
sends a response that indicates its state change. The response includes its session ID in the My
Discriminator field and the session ID of the remote system 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 control packet indicating this state change. This is the third and final part of the handshake.
Now the discriminator values have been exchanged 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 been established. However, because both members must send a control packet — that
requires a response — anytime there is a state change or change in a session parameter, the passive
system sends a final response indicating the state change. After this, periodic control packets are
exchanged.
Figure 14. BFD Three-Way Handshake State Changes
Bidirectional Forwarding Detection (BFD) 181