8 Private VLANs
Subscriber A subscriber C
Router X Router Y
Q-in-Q provider
bridges
One-way
filters
No Yes
Subscriber B
Up VLAN
Down VLAN
2
3
4 1
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router X and router Y. The two routers can speak to each other on the Down VLAN, and
both routers can transmit to both subscribers on the Down VLAN. Note that bridge 3
does not forward data on the Up VLAN down toward bridge 4??”even though there are
subscribers on that bridge??”because of the one-way filters installed on the Up VLAN.
MVRP creates these one-way filters automatically. Each subscriber port, customer port,
and bridge trunk registers the Down VLAN. The propagation of these registrations allow
all ports and bridges to reach each other on the Down VLAN. The Up VLAN, however, is
registered only on the router ports. The propagation of these registrations allows all ports
and bridges to reach the router ports on the Up VLAN, but blocks the Up VLAN in the
direction toward the subscriber ports, where the Up VLAN is not registered.
As far as the routers are concerned, there is exactly one VLAN, the Down VLAN. The
two routers can talk to each other on the Down VLAN.
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