BFD packets flow along the full length of a PW, from T-PE to T-PE. Because they are not intercepted at an S-PE, single-hop initialization procedures are used.
A single BFD session exists per pseudowire.
BFD runs in asynchronous mode.
BFD operates as a simple connectivity check on a pseudowire. The BFD session state is displayed in the MIBs and in the show>service id>sdp>vccv-bfd session command. By default, BFD is not used to change the operational state of the pseudowire, to modify pseudowire redundancy, or to map the BFD state to SAP OAM. However, the router may be optionally configured to take into account the BFD session state.
VCCV BFD runs in software with a minimum supported timer interval of 100 ms for Epipe LDP spoke-SDP; H-VPLS spoke-SDP and mesh-SDP; and Epipe spoke-SDP termination on IES and VPRN interfaces, and inter-chassis backup spoke-SDPs.
BFD is used only for fault detection. While RFC 5885 provides a mode in which VCCV BFD can be used to signal pseudowire status, this mode is applicable only to pseudowires that have no other status signaling mechanism in use. LDP status and static pseudowire status signaling always take precedence over BFD-signaled PW status, and BFD-signaled pseudowire status is not used on pseudowires that use LDP status or static pseudowire status signaling mechanisms.