Any
frame received on the attachment circuit was sent over the outbound LSP, and any
frame received from the inbound LSP was sent over the attachment circuit. The problem
with such a solution, however, is that if a carrier wished to offer Layer 2 services to
its customers, then for each Layer 2 circuit, there was a pair of RSVP-TE LSPs through
the core of the IP/MPLS network, resulting in poor scaling properties for the service.
A more scalable approach was invented by a team working at Level(3) Communications,
and published in IETF as draft-martini (after its inventor, Luca Martini). In the draftmartini
approach, each attachment circuit is associated with a pair of MPLS labels??”
one for each direction??”however these labels are then carried within MPLS LSPs
436 Chapter 14
(using MPLS label stacking), so that a single MPLS LSP can carry a large number of
customer services (and, in fact, the LSPs may be the same ones that are used to carry
IP traffic). The draft-martini approach was eventually standardized in the IETF PWE3
(???Pseudowire Emulation Edge to Edge???) Working Group and forms the basis of the
pseudowire architecture.
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