The two VLANs are the ???Up VLAN???
and the ???Down VLAN,??? as shown in Figure 13.8. The bridges are configured so that on
each subscriber port
?– In the bridge??™s VLAN filtering table, only the Down VLAN is allowed to pass
through the port.
?– All frames emitted from the port (all are on the Down VLAN) are emitted untagged.
?– On ingress, tagged frames are not allowed. (They are discarded.) All incoming
frames are assigned to the Up VLAN.
?– VLAN filtering is disabled for input frames and enabled only for output frames.
The net result of this configuration is that only the Down VLAN can pass out the
port, and all frames coming into the port are accepted and marked with the Up VLAN.
Furthermore, the bridge ports connected to the ISP??™s routers (presumably at least two
of them for redundancy) are configured so that
?– Both the Up and Down VLANs can pass through the ports.
?– Frames in the Up VLAN passing out from the bridge to the router are translated
to the Down VLAN.
Looking at Figure 13.8, you can see that data from subscriber A is tagged with the Up
VLAN. That VLAN is not permitted to exit the port to subscriber B, but can reach both
Figure 13.
Pages:
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887