Understanding Bidirectional Forwarding Detection (BFD)
The Bidirectional Forwarding Detection (BFD) protocol is a simple mechanism that detects failures in a network and works in a wide variety of network environments and topologies. In BFD operation, switches exchanges BFD hello packets at a specified interval and detect a neighbor failure if they do not receive a reply after a specified interval. The BFD failure detection timers support shorter time limits than the static route failure detection mechanisms, so they can provide faster detection of failures.
To configure faster failure detection, use lower BFD timer values. The timers can automatically adapt to a higher value if an adjacency fails, and they also adapt to a higher value when a BFD session flap occurs more than three times in a span of 15 seconds. In this case, a back-off algorithm increases the receive interval by two if the local BFD instance is the reason for the session flap and increases the transmission interval by two if the remote BFD instance is the reason for the session flap.
You can use the clear bfd adaptation command to return BFD interval timers to their configured values. This command is hitless, meaning that it does not affect traffic flow.
QFX10000 series switches support minimum interval values of less than 1 second.
QFX5100 switches and EX4600 switches do not support minimum interval values of less than 1 second.
Starting in Junos OS release 19.4R1, QFX5110, QFX5120, QFX5200, and QFX5210 supports multihop BFD sessions of less than 1 second intervals. 10 inline BFD sessions are supported and can be configured with a timer of 150 x 3 milliseconds. Inline BFD is enabled by default.
Starting in Junos OS release 19.1R1, QFX5120-32C supports BFD sessions of less than 1 second intervals. Performance might vary depending on the configuration load within the system.
Starting in Junos OS release 18.4R2.S3, QFX5110 and QFX5120 switches support up to 8 sub-second BFD sessions and can be configured with a timer of 150 x 3 milliseconds.
On EX2300 and EX3400 switches, the BFD protocol must be configured with a Hello interval of 2 seconds or more with dead interval not less than 6 seconds to prevent flaps during CPU intensive operations events such as routing engine switchover, interface flaps, and exhaustive data collection from the packet forwarding engine.
BFD is not supported with ISIS for IPv6 on QFX10000 series switches.
EX3300 supports BFD over static routes only.