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Dell EMC SmartFabric OS10 User Guide Release 10.5.1

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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 session state changes

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