392 Chapter 13
Provider Bridge Solutions and Challenges
Some very substantial provider networks have been implemented using the Q-in-Q
technology of 802.1ad. However, there are limitations:
?– The provider??™s bridges learn the customers??™ MAC addresses. If a substantial number
of customers are using their EVCs to carry bridged traffic, so that the number
of customer MAC addresses is large, the total number of MAC addresses over all
customers can exceed the limitations of most bridge implementations.
?– A VLAN tag allows for only 4094 EVCs. A large provider needs to offer many more
EVCs than this in a single network.
?– A network serving all of a provider??™s customers can have more bridges than the
802.1 spanning tree algorithms can support and still maintain adequate response
times to failure conditions.
Some of these limitations have workarounds, and some do not.
Number of Customer MAC Addresses The problem of Q-in-Q provider bridges having to
learn too many customer MAC addresses can be solved in several ways. The most obvious
is for the provider to charge for customers who require more than two or three MAC
addresses per port.
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