They can send data, whether
broadcast, multicast, or unicast, to the subscribers. The VLAN looks to the routers like
an ordinary subnet with, in this example, 500 hosts. The subscribers can talk to the
routers with broadcasts, multicasts, or unicasts. Their frames, input on the Up VLAN,
are delivered to the router(s) as if they were on the Down VLAN. The only catch is that
the subscribers cannot talk to each other at all. Any frame from a subscriber is on the
Up VLAN and thus cannot be emitted on any other subscriber??™s port. Any subscriberto-
subscriber traffic must pass through the router. (In IPv4 terms, the routers have
the correct IP network mask, and the subscribers are told that the network mask is
255.255.255.255, so they send all IP packets to the router.)
With private VLANs, hundreds of subscribers can be served with one pair of VLANs,
and up to 2047 VLAN pairs are available. In this way, a Q-in-Q Carrier Ethernet network
can connect a sufficient number of subscribers to satisfy any carrier.
Private VLANs cannot, however, solve the problem of private networks for customers,
such as businesses typically expect. In these instances, only one VLAN per customer
will work, and the 4094 limit can be a problem.
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