Most commonly replies are sent as IP packets,
but they may be sent as IP packets with the router alert option set (which forces the
packet to the control plane of each router in the path, and which may therefore enable
a response to reach its target in the presence of forwarding plane bugs on the return
path, but does require all routers on the return path to implement LSP ping), or using an
associated control channel, such as VCCV (see the section on Pseudowire OAM below).
The LSP ping request also carries a copy of the FEC being tested, so the egress LSR may
verify that it is an egress for that FEC in both the control and forwarding planes.
If proactive detection of forwarding-plane faults is required, then a lighter-weight
protocol than ICMP ping (or LSP ping) is required. The IETF-defined forwarding-plane
OAM mechanism for IP (and for MPLS) is Bidirectional Forwarding Detection (BFD).
BFD may be used for rapid detection of routing/signalling neighbour failure in cases
where the router fails to detect loss of the physical layer, and may also be applied
end-to-end on MPLS LSPs to detect LSP failure, as discussed below in the section
on OAM-based Fault Detection.
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