That is what I found in a cisco document. makes it more clear?
In a typical deployment of VLAN mapping, you want the service provider to provide a transparent
switching infrastructure that treats cu
stomers’ switches at the remote loca
tion as a part of the local site.
This allows customers to use the same VLAN ID space and run Layer 2 control protocols seamlessly
across the provider network. In such scenarios, we recommend that service providers do not impose their
VLAN IDs on their customers.
One way to establish translated VLAN IDs (S-VLANs)
is to map customer VLAN
s to service-provider
VLANs (called VLAN ID translation) on trunk ports
connected to a customer
network. Packets entering
the port are mapped to a service
provider VLAN (S-VLAN) based on the port number and the packet’s
original customer VLAN-ID (C-VLAN).
Service providers’s internal assignments might confli
ct with a customer’s VLAN. To isolate customer
traffic, a service provider could decide to map a sp
ecific VLAN into another one while the traffic is in
its cloud.
