Layer 3 protocols can use Bidirectional Forwarding Detection (BFD) for rapid failure detection in the forwarding path between two adjacent routers, including the interfaces, data links, and forwarding planes.
BFD can be configured for use with the following protocols:
BFD must be enabled at the interface and routing protocol levels. BFD asynchronous mode depends on the sending of BFD control packets between two systems to activate and maintain BFD neighbor sessions between routers. Therefore, BFD must be configured on both BFD peers.
A BFD session is created after BFD is enabled on the interfaces and at the router level for the appropriate routing protocols. BFD timers are then negotiated, and the BFD peers begin to send BFD control packets to each other at the negotiated interval.
BFD provides one point of forwarding path monitoring when more than one Layer 3 application wants to monitor a host. BFD runs a session for that host and provides the status to multiple applications, instead of multiple applications running individual sessions to the host.
By sending rapid failure detection notices to the routing protocols in the local device to initiate the routing table recalculation process, BFD contributes to greatly reducing overall network convergence time.
Note
BFD, IS-IS, and OSPF stop operating when Rapid Spanning Tree Protocol (RSTP) path-cost changes are made to the Alt Discarding port on the switch.The following figure shows the establishment of a BFD session where OSPF discovers a neighbor and sends a request to BFD requesting that a BFD neighbor session be created with the OSPF neighbor router.
The following figure shows the termination of a BFD neighbor session after a failure occurs in the network.
