At the end of the LSP, the label
stack is popped either by the egress LSR or by the penultimate router (the latter
mode is known as penultimate hop popping or PHP and is permitted in MPLS
because it reduces the load on the egress LSR in some cases). A pop operation
may also instruct the LSR to process the next label??”for example at intermediate
LSRs along an LSP, a label may be popped before swapping the next label.
MPLS Signalling
MPLS LSPs may be established using various methods. The most commonly used are
covered in this section.
Static Configuration The operator can provision LSPs through the network by statically
configuring label mappings at each LSR. This can be labour intensive, of course,
and unless OAM flows are sent (see the section on MPLS Protection below), it also creates
an unprotected LSP; if any of the nodes or links on the paths fail, the LSP will no
longer forward from ingress to egress.
MPLS 425
LDP Signalling The Label Distribution Protocol (LDP) is documented in RFC 3036
(though the specification is in the process of being revised). It was also extended to offer
Constraint-Based Routing as CR-LDP in RFC 3212 but CR-LDP lost out in the market
to RSVP-TE (see the description of RSVP-TE in the next section).
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