API Guide

BFD session states
To establish a BFD session between two routers, enable BFD on both sides of the link. BFD routers can operate in both active
and passive roles.
● The active router starts the BFD session. Both routers can be active in the same session.
● The passive router does not start a session. It only responds to a request for session initialization from the active router.
A BFD session can occur in Asynchronous and Demand modes. However, OS10 BFD supports only Asynchronous mode.
● In Asynchronous mode, both systems send periodic control messages at a specified interval to indicate that their session
status is Up.
● 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.
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.
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.
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