BFD has a simple packet format and is designed to
be implemented in the router??™s forwarding engine, rather than in the control plane.
However, BFD is unable to support traceroute functionality or to verify the mapping
between the control and forwarding planes at egress (hence LSP ping is still a useful
tool for problem diagnosis in the event that BFD detects a problem).
When sent over MPLS LSPs, BFD is used in asynchronous mode, where the ingress
LSR periodically sends BFD control packets to the egress LSR over the LSP being
tested, and the egress LSR periodically sends BFD control packets to the ingress LSR.
If the egress LSR fails to receive a BFD packet from the ingress LSR in a negotiated
multiple of the period at which the ingress LSR sends them, it sends a BFD packet
identifying the fault condition. The ingress LSR sends BFD packets to a destination
address in 127/8 and to UDP port 3784, setting the IP TTL to 1. The egress LSR sends
BFD packets to the same IP address that the ingress LSR sources its packets from and
from UDP port 3784.
When using BFD to detect MPLS LSP failures in conjunction with MPLS fast reroute
(see section below on fast reroute), it is essential to tune the frequency with which BFD
packets are sent so that fast reroute repairs problems faster than BFD detects them.
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