BFD stands for Bidirectional Forwarding Detection protocol
It’s designed to detect failures in the forwarding path between network devices such as routers and switches. BFD works by sending periodic probe packets between the endpoints of the connection, and if the endpoint doesn’t receive a response to a certain number of probe packets within a specific time period, it assumes the connection has failed and takes the necessary action.
The BFD protocol is defined in RFC 5880 and is fast, lightweight, and has low overhead, making it an ideal solution for network performance monitoring. In Cisco SD-WAN, BFD serves two critical purposes: liveliness detection and path quality monitoring for loss, latency, and jitter.
BFD is enabled by default on Cisco WAN Edge routers between peers, running across all IPsec tunnels and transports. BFD operates in “echo mode,” which means that when BFD packets are sent by a WAN Edge router, the receiving WAN Edge router returns them without processing them. BFD is used to detect both black-out and brown-out scenarios.
BFD hello packets are sent every 1000 milliseconds (1 second) on every tunnel interface, and the default BFD multiplier is 7, meaning the tunnel is declared down after 7 consecutive hellos are lost. BFD packets are marked with a DSCP value of CS6 (48 DSCP), and the WAN Edge router collects packet loss, latency, and jitter information for every BFD hello packet. The BFD hello interval and multiplier are configurable on a per-color basis.
However, it’s worth noting that only single-hop BFD is supported, and BFD session modes (from echo to no echo and vice versa, or from software to hardware and vice versa) don’t update immediately after changing the BFD template parameters in Cisco vManage. To change the BFD session modes, you need to remove all existing BFD configuration and reconfigure it. Only supported for BGP, EIGRP, OSPF, and OSPFv3 (Cisco SD-WAN Routing Protocols)
Overall, BFD plays a crucial role in maintaining network performance and detecting failures in real-time, making it an essential tool for any network administrator to have in their toolkit.