Figure 2: BFD Process During a Network Failure A routing protocol must register with BFD for every neighbor it acquires. Once a neighbor is registered, BFD initiates a session with the neighbor if a session does not already exist. OSPF registers with BFD when: • A neighbor finite state machine (FSM) transitions to full state. • Both OSPF BFD and BFD are enabled. On broadcast interfaces, OSPF establishes a BFD session only with the designated router (DR) and backup designated router (BDR). The session is not established between any two devices in a DROTHER state. BFD Detection of Failures Once a BFD session is established and timer negations are complete, BFD peers send BFD control packets.
The packets act in the same manner as an IGP hello protocol to detect liveliness, except at a more accelerated rate. The following information should be noted: • BFD is a forwarding path failure detection protocol. BFD detects a failure, but the routing protocol must act to bypass a failed peer. • Starting with Cisco IOS XE Denali 16.3.1, Cisco devices support BFD version 0. Devices use one BFD session for multiple client protocols in the implementation. For example, if a network is running OSPF and EIGRP across the same link to the same peer, only one BFD session is established. BFD shares session information with both routing protocols.
BFD Version Interoperability All BFD sessions come up as Version 1 by default and are interoperable with Version 0. The system automatically performs BFD version detection, and BFD sessions between neighbors run in the highest common BFD version between neighbors. For example, if one BFD neighbor is running BFD Version 0 and the other BFD neighbor is running Version 1, the session runs BFD Version 0. The output from the show bfd neighbors [ details ] command verifies which BFD version a BFD neighbor is running. See the "Example Configuring BFD in an EIGRP Network with Echo Mode Enabled by Default" for an example of BFD version detection.
BFD Session Limits Starting with Cisco IOS XE Denali 16.3.1, the number of BFD sessions that can be created has been increased to 100. Configuring Bidirectional Forwarding Detection BFD Detection of Failures Configuring Bidirectional Forwarding Detection 3