Provider bridged networks can be interconnected via backbone clouds in two different
ways. In Figure 13.1, clouds D, E, F, and H are connected in the most typical manner.
376 Chapter 13
Up to 16 million (224) individual EVCs can traverse the backbone core. (More, if you
are willing to accept the headache caused by spatial reuse of identifiers.) Each of the
4094 EVCs in a given provider bridge network is translated to one of the 16M backbone
instances at the edge of the backbone network. Those EVCs are, in turn, tunneled inside
an outer encapsulation that enables the backbone network to see only tunnels containing
1 or 1000s of EVCs, instead of seeing individual EVCs. An EVC can be connected
in an arbitrary fashion to any number of provider bridge networks, and each provider
bridged network??™s VID is independent of any other cloud??™s VID.
Alternatively, two provider bridge networks, C1 and C2 in Figure 13.1, can tunnel
through the backbone cloud using a single 16M backbone EVC. In this case, clouds C1
and C2 are in the same logical relationship as clouds F and G; they are simply using
the backbone cloud, instead of a physical link, for their connectivity.
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