Hi,
VLAN stacking is also called "802.1q tunneling" or "802.1q in 802.1q" or simply q-in-q. They all refer to the same thing.
The idea behind this technology is to allow service providers to provide transparent LAN services to customers. The VLAN ID is a 12-bit field, restricting you to 4096 VLAN IDs. IF a provider wanted to offer transparent ethernet services, the number of VLAN IDs it could use would quickly run out if it mapped a provider VLAN ID to every customer VLAN ID. The use of q-in-q means that you can use a single VLAN ID per customer, within which you could carry the full range of customer VLAN IDs. Theoretically, you could scale up to 4096*4096 VLAN IDs !! As an example, say a customer with VLANs 1, 2 , and 3 wants a transparent LAN service. The provider would allocate it a VLAN, say 100, within its core. Customer traffic would enter the provider network as tagged frames. The provider would then attached another tag on top of that, the one for VLAN 100. Therefore, the frames would have the following tags within the core: (100 1), (100 2) and (100 3). When switching within the core, the provider would only look at the top tag, 100. When a frame egresses the provider network, the provider strips the top tag, leaving the original VLAN tag in place.
The following link may help:
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12113ea1/3550scg/swtunnel.htm
THe following document is also quite good:
http://www.nettonet.com/solutions/appnotes/an-vlanstack.pdf
Hope that helps - pls rate the post if it does.
Paresh
