The other is for the customer to purchase two separate EVCs in two separate
S-VLANs and make sure no address is duplicated in either S-VLAN. In this case, the
dotted C-VLAN could stay in the same S-VLAN, and the solid C-VLAN could be moved
to a new S-VLAN. If the customer is unaware of this possibility, however, it can take
some effort to resolve and can involve some avoidable finger pointing.
Although this problem condition is easy for a customer to avoid, it does point out
why you cannot easily build a ???Q-in-Q-in-Q??? bridge (or a ???4-Q??? or a ???5-Q??? bridge). One
customer can adjust his or her configuration to avoid this problem. But, adding a third
VLAN tag means that two or more different customers??™ EVCs are packed inside a single
outermost VLAN tag. A provider cannot expect those customers, perhaps all running
VRRP, to cooperate in their network configurations in order to avoid this duplicate address
problem. This is especially true since, if one of those customers??™ network administrators
makes a configuration error, all of the customers will suffer from the consequent
misdirection of frames.
Proper Layering Why can??™t a bridge simply look at two tags and differentiate between
the two routers? The answer is layering.
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