Reference Guide

Bidirectional Forwarding Detection | 173
BFD sessions
BFD must be enabled on both sides of a link in order to establish a session. The two participating systems
can assume either of two roles:
• Active—The active system initiates the BFD session. Both systems can be active for the same session.
• Passive—The passive system does not initiate a session. It only responds to a request for session
initialization from the active system.
A BFD session has two modes:
• Asynchronous mode—In Asynchronous mode, both systems send periodic control messages at an
agreed upon interval to indicate that their session status is Up.
• Demand mode—If one system requests Demand mode, the other system stops sending periodic
control packets; it only sends a response to status inquiries from the Demand mode initiator. Either
system (but not both) can request Demand mode at any time.
A session can have four states: Administratively Down, Down, Init, and Up.
• Administratively Down—The local system will not participate in a particular session.
• Down—The remote system is not sending any control packets or at least not within the detection time
for a particular session.
• Init—The local system is communicating.
• Up—The 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 handshake
A three-way handshake must take place between the systems that will participate in the BFD session. The
handshake shown in Figure 9-2 assumes that there is one active and one passive system, and that this is the
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 passive system responds. These packets are sent at the desired transmit interval of the Active
system, and 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 of the
Note: FTOS supports asynchronous mode only.