When looking at
how a service provider can implement a metro Ethernet network, try starting with the
question, ???Can you scale an enterprise network to one supporting millions of subscribers
spread over a continent???? This is a problem that a number of bridge vendors have
addressed successfully and that the IEEE 802.1 Higher-Layer Interworking Group has
been standardizing.
Figure 13.1 shows the architecture. The overall network consists of independent
clouds of bridges. There are two kinds of clouds, provider bridged networks (clouds C
through H) and provider backbone bridged networks (clouds A and B). Each provider
bridged network supplies up to 4094 instances of the MAC service to customers, each
with an Ethernet Virtual Connection (EVC).
Provider bridged networks can be linked together, as are clouds F and G. Within each
cloud, there are 4094 service instances (EVCs), but only those EVCs that span both
clouds require VLANs (each identified by a VLAN ID (VID)). VLANs for EVCs that
span only a single cloud can be reused in the attached cloud. For example, you could
use 1000 VIDs for VLANs spanning both clouds F and G, leaving 3000 VIDs for local
use in each cloud F and G, for a total of 7000 VLANs.
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