Existing aggregation networks belonging to the service provider or to third
party carriers (often regional or national incumbents) and comprised of devices such
as Ethernet switches, SONET/SDH MSPPs and DXCs, PDH muxes, and DSLAMs are
used to aggregate the customer traffic. The connection from the aggregation network to
the MPLS PE may be an Ethernet UNI (i.e., one physical connection per customer), an
Ethernet NNI (i.e., multiple customers delivered on one physical Ethernet connection),
or may be in the native format of the aggregation network (e.g., channelised SONET/
SDH with a GFP-encapsulated VCAT group per customer). In this case, the value of
MPLS is that it enables the service provider to offer Layer 2 Ethernet services between
Figure 14.6 Large MPLS metro network
Ethernet
switch
layer
U-PE
layer
N-PE
layer
Core
layer
454 Chapter 14
cities (or within large cities) and also to offer Layer 3 services from Ethernet concentration
nodes that aren??™t connected directly to a local Layer 3 service PE.
Multiple Services on One Network (Not Just Ethernet)
As discussed in the section on network convergence, MPLS is the natural technology
choice for offering a range of Layer 1, Layer 2, and Layer 3 services over a common network.
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