BFD, however, may be used as a proactive tool for pseudowire monitoring??”though
since routers can generally support only a relatively small number of BFD sessions,
VCCV BFD is rarely enabled except for very high value pseudowires or where potential
issues have already been identified. VCCV BFD runs in asynchronous mode, with each
PE sending BFD packets to the remote PE. If either PE misses a negotiated number of
BFD packets from its peer, it will send a BFD packet indicating that it cannot ???hear??? its
packets. Both PEs will (if possible) notify their attached CEs of the fault condition.
Ultimately, even with VCCV, the OAM mechanisms only test the pseudowire itself;
they do not test the adaptation function that encapsulates Layer 2 frames into pseudowire
packets or the functioning of the attachment circuit itself. For this reason, it
may be useful for the PE to perform client-layer OAM tests across the pseudowire and
toward the attachment circuit.
Pseudowire Redundancy A pseudowire connects two PE devices over an MPLS network.
However, there are cases where providers want to provide backup for a pseudowire.
MPLS 441
One example might be when connecting a DSLAM to two BRAS devices so that the
subscriber??™s broadband service can recover from BRAS failures.
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