Now, these two devices
can communicate with no flooding involved.
Ethernet Bridging 399
Advantages of Backbone Bridges From the previous rather complex example, you can
see why backbone bridging is attractive. The most important result is that the number
of EVCs has been brought up from 4094 to 16 million. The second result is less obvious.
Only those I-components that handle a particular EVC need to learn the customers??™
MAC addresses. No B-component had to learn any customer MAC addresses. Therefore,
all learning of customer MAC addresses is isolated. The backbone bridges X and Y in
Figure 13.10 know nothing at all of the customer??™s MAC addresses. Each I-component
knows, at most, the MAC addresses of only the 4k EVCs it serves and no others. Of
course, the schemes for reducing the MAC addresses learned for provider bridges are
available to the I-components, since they are, themselves, provider bridges.
In particular, EVCs known to be E-Lines can be handled specially. For example, if an
I-component knows that an EVC is an E-Line, then it does not need to learn any customer
MAC addresses; it only needs to learn the MAC address of the one I-component at
the other end of the EVC.
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