?– Networks where different pseudowire signalling and encapsulation techniques are
used in different areas of the network (for example, when interoperating between FEC
128 and 129 pseudowire signalling or between RSVP-TE and LDP-signalled LSPs).
?– Pseudowires established between different service providers where the two service
providers would rather not allow each other to set up LDP sessions and MPLS
LSPs directly from PE to PE.
In the segmented pseudowire architecture, a pair of Terminating PEs (T-PEs) may
establish a pseudowire to each other via one or more Switching PEs (S-PEs). Each S-PE
connects two PW segments??”and performs the necessary signalling and encapsulation
for each one.
Forwarding along a multi-segment pseudowire (or MS-PW) is the same as forwarding
along an LSP, except that the S-PE performs a label swap on the PW label, rather than
the tunnel label. When a fault is detected in a segment of an MS-PW, LDP is used to
propagate that fault information toward the T-PEs (or, in the case of a fault detected at
one T-PE, to propagate that information to the remote T-PE).
Additional LDP TLVs have been defined to enable the path of MS-PWs to be traced
in the control plane, as have extensions to VCCV to enable the path to be traced in the
forwarding plane (and to enable fault detection in the forwarding plane).
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