Such a network will be practical only when bridges acquire the capability of offering
a large number of point-to-point connections with very fast (on the order of 10s of
milliseconds) failure recovery capabilities. This capability is still in development.
Sparse Wide Area Business Services
Given an underlying network of point-to-point services, whether MPLS, SONET, or
ATM, you can create very simple E-Line EVCs such as MPLS pseudowires. Provider
bridges can use both physical Ethernet links and these virtual links equally well for
connecting to customers and to each other. For example, taking just one of the IEEE
802.1ad provider bridge Q-in-Q clouds from Figure 13.1, up to 4094 E-LAN business
EVCs can be created and overlaid upon the MPLS network. MPLS can continue to be
used for all E-Line EVCs, including Ethernet-over-MPLS services.
Ethernet Bridging 415
Metro Area Business Services
A single IEEE 802.1ad provider bridge Q-in-Q cloud can offer 4094 business EVCs,
enough for many metropolitan networks, without recourse to a MAC-in-MAC backbone.
As the network grows and the limitations of this Q-in-Q network are approached, it can
be split in two, using VID translation.
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