Ideally VLAN ID Preservation should be preserved ( CE-VLAN ID = Yes). After all, why it should matter, changing VLAN IDs. Having same VLANs IDs make it operationally efficient for both operator and customer. Operator can trace the circuit well, so can the end customer by just knowing the VLAN IDs. Not to forget that it would be easy to setup such an EVC since the VLAN ID to EVC map will be much simple in this case.
In this post I will explain that why it is needed to set VLAN Preservation ID = NO
Let’s see the example of setting VLAN Preservation ID = YES and its advantages. See the diagram 1 below, customer has two campuses with the same VLAN number 14. Obviously this situation demands that VLAN Preservation ID should be set to YES. Why asking the end customer to change VLAN IDs in the first place ?
Lets see a second scenario in which two campuses of the same customer are connected to each other but both have different VLANs. The obvious advantage here for using VLAN Preservation ID= No is to let the service provider do the switch and hence in that case the service provider will switch the VLAN ID from 15 to 14 before handing the traffic to B while the converse will be true when handing the traffic to A. This is shown in diagram 2 below.