Connectivity 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 specied 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 notication it receives from the peer router. For example, if the current
session state is Down and the router receives a Down status notication from the remote router, the session state on the local router
changes to
Init.
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