Users Guide

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.
BFD session state changes example
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.
NOTE: BFD sessions flap when the node has multiple unresolved IPv6 PTP slaves and hence Dell EMC recommends running
one of the protocols in the node. This issue exists only with the IPv6 slaves.
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 exchange, and transmit intervals negotiate.
4. The passive system receives the control packet and changes its state to Up. Both systems agree that a session is
established. However, because both members must send a control packet, which requires a response, whenever there is a
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