Connectivity Guide

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 rst 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 eld 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 eld and the session ID of the remote system in the Your
Discriminator eld.
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 state change or change
in a session parameter, the passive system sends a nal response indicating the state change. After this, periodic control packets
exchange.
BFD conguration
Before you congure BFD for a routing protocol, rst enable BFD globally on both routers in the link. BFD is disabled by default.
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Layer 3