Reference Guide

● In Demand mode, if one router requests Demand mode, the other router stops sending periodic control packets; it only sends
a response to status inquiries from the Demand mode initiator. Either peer router, but not both, can request Demand mode
at any time.
A BFD session can have four states: Administratively Down, Down, Init, and Up. The default BFD session state is Down.
● Administratively Down — The local BFD router does not participate in the session.
● Down — The remote BFD router is not sending control packets or does not send them within the detection time for the
session.
● Init — The local BFD outer is communicating to the remote router in the session.
● Up — Both BFD routers are sending control packets.
A BFD session's state changes to Down if:
● A control packet is not received within the detection time.
● Demand mode is active and a control packet is not received in response to a poll packet.
Example: BFD session state changes
The session state on a router changes according to the status notification it receives from the peer router. For example, if the
current session state is Down and the router receives a Down status notification from the remote router, the session state on
the local router changes to Init.
BFD three-way handshake
A BFD session requires a three-way handshake between neighboring routers. In the following example, the handshake assumes:
● One router is active, and the other router is passive.
● 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 to indicate 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 a control packet, it changes its session state to Init and sends a response to indicate
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 to indicate this state change. Discriminator values are exchanged, and transmit intervals are negotiated.
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