6 is that, because both of the customer
bridge ports facing the EVC have equal costs to reach the root bridge in the left cloud,
neither of those two ports will be blocked. Some other port on one of those bridges will
be blocked in order to prevent endless forwarding loops. Therefore, those stations in the
right cloud connected directly to the bridge whose port is blocked can only reach the
other parts of the right cloud via the provider network!
Again, this problem has solutions. See ???Bridge Gateways,??? later in the chapter.)
C-tagged Interfaces A number of considerations led to the standardization of C-tagged
interfaces in IEEE Std 802.1ad-2005. One was the port blocking problem shown in
Figure 13.6. The other was the desire of providers to offer more than just the port-based
EVC described so far, wherein everything entering the provider bridge port is assigned
to the same S-VLAN in the provider network. With the C-tagged interface, the customer
can select from among any number of EVCs (up to 4094) using C-tags.
Figure 13.7 shows a simple example of complex EVCs, with two customers attached
to a single provider edge bridge.
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