Bfd Sessions; Bfd Three-Way Handshake - Dell S4048T Configuration Manual

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 In Asynchronous mode, both systems send periodic control messages at an agreed
mode 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: Dell Networking OS supports Asynchronous mode only.
A session can have four states: Administratively Down, Down, Init, and Up.
State Description
Administratively The local system does not participate in a particular session.
Down
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.
Bidirectional Forwarding Detection (BFD) 180