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.
NOTE: The Dell Networking OS supports Asynchronous mode only.
A session can have four states: Administratively Down, Down, Init, and Up.
Administratively
Down
The 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 particular session.
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 Handshake
A 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 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. 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
Bidirectional Forwarding Detection (BFD) 127