As these
networks grew, various challenges emerged:
?– How to scale the network within each city
?– How to improve the availability of the network, which was often far below that of
traditional SONET/SDH networks
?– How to link Metro Ethernet networks in different cities together
?– How to provide access outside the major city centres and in locations that were
off-net for fibre
?– How to offer differentiated service offerings, especially in the case of services with
enhanced SLAs for availability and QoS
Many service providers have opted to address these five challenges by deploying
large MPLS PE devices in the core of each Metro Ethernet network (see Figure 14.11).
Each network may then be subdivided into smaller Layer 2 domains, improving scaling
by segmenting the VLAN space, reducing the number of MAC addresses learned on
each Ethernet switch, and reducing the size of spanning tree protection domains. The
PE devices themselves are typically larger and more resilient than the core Ethernet
switches they replace??”both increasing the scale of the network and improving its
Figure 14.10 Using H-VPLS for broadcast TV forwarding
MPLS core
P2MP LSP
IPTV feed
U-PE N-PE
DSLAM
Ethernet PW
MPLS 463
availability.
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