Because many customers??™ EVCs connect routers to those services, this
is not a problem for most customers. If customers who use a provider network to connect
their bridges together are charged extra for burdening the provider with a large number
of MAC addresses, then the provider can afford more memory for MAC address tables.
A second way to mitigate the problem is provided by IEEE Std 802.1ad-2005. If the
provider bridges are running MVRP, they can prune back each S-VLAN so each EVC
goes only to those bridges in the network that are required to carry it. If a given EVC is
an E-Line, with only two endpoints, then every bridge along its path will have exactly
two ports that are not pruned for that VLAN??”the two that are necessary to carry that
VLAN from endpoint to endpoint. Any bridge that has only two active ports can disable
MAC address learning for that VLAN. If many of the EVCs are E-Lines, doing this can
greatly reduce the number of customer MAC addresses learned.
Furthermore, because each VLAN follows a spanning tree, on an E-LAN EVC with
three customer ports, exactly one bridge in the network will have three ports on that
EVC; all other bridges will have, at most, two active ports.
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