Bidirectional Forwarding Detection (BFD) is a detection protocol that is designed to detect forwarding path failures in less than one second. Additionally, BFD is designed to work regardless of media type, encapsulation, or routing protocol, providing network administrators with a uniform forwarding failure detection method across a network. BFD supports Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol (BGP), and Intermediate System-to-Intermediate System (IS-IS).
BF has two operating modes: Asynchronous mode and Demand mode. Asynchronous mode systems periodically send BFD Control packets. If Control packets are not received from a neighbor in a timely fashion, the neighbor is assumed to be down.
Demand mode systems assume that there is an external method of verifying connectivity. When Demand mode is enabled on a system, the system can ask neighbors to stop sending BF Control packets except when absolutely necessary. This enables Demand mode systems to support a large number of BFD sessions, but the relative lack of Control packets can cause failure detection to be slower.
The Echo function can be enabled with Asynchronous mode or Demand mode. When the Echo function is enabled on a system, the system will send a stream of BFD Echo packets to a neighbor. If enough Echo packets are not returned, the neighbor is assumed to be down. The Echo function can reduce round-trip jitter and can increase the speed of failure detection. Although the Echo function does not send packets as often as Asynchronous mode does, Asynchronous mode requires half as many packets as the Echo function does in order to detect a failure.