Bidirectional Forwarding Detection (BFD) is a unified detection mechanism used to rapidly detect link faults. BFD improves network performance by providing fast forwarding path failure detection times.
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Hardware-assisted BFD helps support multiple BFD sessions.On SLX 9150 and SLX 9250 series devices, hardware-assisted BFD is supported over IPv4 and IPv6.
BFD provides rapid detection of the failure of a forwarding path by checking that the next-hop device is alive. When BFD is not enabled, it can take time to detect that a neighboring device is not operational. This causes packet loss due to incorrect routing information at a level unacceptable for real-time applications such as VoIP and Video over IP.
Using BFD, you can detect a forwarding path failure in 150 milliseconds.
A BFD session is automatically established when a neighbor is discovered for a protocol, provided that BFD is enabled on the interface on which the neighbor is detected and BFD is also enabled for the protocol at the interface level or globally. After a session is established, each device transmits control messages at a high rate of speed that is negotiated by the devices during the session setup.
To provide a detection time of 150 milliseconds, it is necessary to process 20 messages per second of about 70 to 100 bytes each per session. A similar number of messages also need to be transmitted out per session. After a session is established, that same message is continuously transmitted at the negotiated rate and a check is made that the expected control message is received at the agreed-upon frequency from the neighbor. If the agreed-upon messages are not received from the neighbor during a negotiated timeout period, the neighbor is considered to be down.
BFD can provide failure detection on any kind of path between systems, including direct physical links and multihop routed paths. Multiple BFD sessions can be established between the same pair of systems when multiple paths between them are present in at least one direction, even if a lesser number of paths are available in the other direction.
For a single-hop session, the BFD Control Message is a UDP message with destination port 3784. For a multihop session, the BFD Control Message is a UDP message with destination port 4784 sent over IPv4 or IPv6, depending on which data forwarding path failure BFD is trying to detect.
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