?– Insertion of a router alert label between the tunnel label and the PW label, known
as out-of-band VCCV. Because of the change to the label stack, this approach may
result in OAM packets taking a different path to data packets when ECMP is used.
?– TTL expiry??”by setting the TTL in the PW label to 1, the ingress PE can force the
egress PE to process the packet in the control plane. Again, in this mode, the control
word must be present.
The key in all three control channel cases is that VCCV packets are sent along the
same LSP, and using the same PW label, as user data on the pseudowire, but are intercepted
by the control plane of the egress PE rather than being forwarded on the egress
attachment circuit.
LSP ping over VCCV may be used as a reactive troubleshooting tool to verify that the
pseudowire is passing traffic correctly. LSP ping packets identify the FEC (FEC 128 or
129) for the pseudowire. Since pseudowires only have a single-hop LSP, traceroute is
not used. In the VCCV case, the PE sending the LSP ping will typically indicate that
it wants the reply sent over the reverse path of the pseudowire, but if the ping fails, it
may try other response methods to identify whether the problem is on the forward or
reverse direction of the pseudowire.
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