Connectivity Guide
BFD provides forwarding-path failure detection in milliseconds instead of seconds. Because BFD is independent of routing protocols, it
provides consistent network failure detection. BFD eliminates multiple protocol-dependent timers and methods. Networks converge is
faster because BFD triggers link-state changes in the routing protocol sooner and more consistently.
BFD is a simple hello mechanism. Two neighboring routers running BFD establish a session using a three-way handshake. After the session
is established, the routers exchange periodic control packets at sub-second intervals. If a router does not receive a hello packet within the
specied time, routing protocols are notied that the forwarding path is down.
In addition, BFD sends a control packet when there is a state change or change in a session parameter. These control packets are sent
without regard to transmit and receive intervals in a routing protocol.
BFD is an independent and generic protocol, which all media, topologies, and routing protocols can support using any encapsulation. OS10
implements BFD at Layer 3 (L3) and with User Datagram Protocol (UDP) encapsulation. BFD is supported on static and dynamic routing
protocols, such as VRRP, OSPF, OSPFv3, IS-IS, and BGP.
The system displays BFD state change notications.
NOTE: BFD is only supported for the border gateway protocol (BGP).
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 specied 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 notication it receives from the peer router. For example, if the current
session state is Down and the router receives a Down status notication from the remote router, the session state on the local router
changes to Init.
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