These two limitations mean that the C-component never has to learn MAC addresses.
Each frame that passes from the S-component to the physical link has its S-tag stripped,
or perhaps (e.g., in the case of customer 1s C-VLAN 18) translated to a C-tag. On input,
every frame is either discarded, wrapped in an S-tag, or translated to an S-tag.
The C-component also has the ability to translate the C-tags??™ priority fields into
appropriate values for the priority fields in the S-tags and may be able to filter (not
output) selected C-VIDs in the direction from the provider to the customer. Thus, it is
an exceptionally simple bridge, consisting simply of Q-tag translation tables and not
requiring MAC address inspection.
As a VLAN bridge, however, the C-component does participate in the customer??™s
bridge control protocols such as spanning tree, 802.1X port authentication, and/or
MVRP. If the provider is willing to accept the configuration requirements to participate
fully in the customer??™s spanning tree, the customer??™s data can be controlled easily. (Note
that these customer BPDUs pass transparently through the S-components.) However,
much simpler, and much better, alternatives are presented in ???Bridge Gateways,??? later
in the chapter.
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