Pre-requiste: Understanding of the 802.1Q Protocol
The purpose of this post is to shed a light on how QinQ Vlan takes place in a bridged network environment.
Before continuing, it is important to keep in mind that 802.1QinQ or 802.1ad isn’t a defined protocol in itself but a mere amendment of the already existing 802.1Q protocol.
Having said that, in a nutshell where a single frame can hold a maximum of 4096 tags, QinQ extends the number of maximum tags to 16777216 tags, thus allowing switches to freely manipulate the tags of a single packet. A typical example where QinQ is used is in bridge networks where each customer’s frame can be easily forwarded through different topology network while appearing to the customer’s as a simple bridge with no frame modification.
That is to say, if a corporation has different offices across a region and wishes to build a single logical lan, the corporation can use QinQ and bridge all its sites through their subscribed network provider, without having to alter the existing vlan infrastructure of the customer.
This as said earlier is achievable through QinQ and S-Vlans. To keep it simple, S-Vlans are just the vlan tags that the frames of a customer gets when entering the vlan space of the Service provider and on which forwarding occurs.
Office A is on vlan 1 —- Provider — Office B is on vlan 1
For this to work such as the packet from customerA tagged with vlan 1 be tunneled through the ISP’s bridged network, the ISP must work on a different vlan space and assign a specific S-Vlan ID to the coorporation subscribed to its services.
Office A (vlan1) —- Provider (vlan20) — Office B (vlan 1)
When entering the Provider’s bridge, the frame from OfficeA will be tagged with S-Vlan 20 and be forwarded to OfficeB, once the packet reaches the other edge bridge’s endpoint, it is stripped off the S-vlan and enters the office’s B network.
Now what if I have many vlans? Remember, within the Bridged network, the Vlans aren’t looked at, only the S-vlan is looked at… based on the S-vlan, the provider’s switch makes a decision as to which S-vlan switch end point to forward to, to which the customer-network port is assigned. Only once it arrives at the other endpoint, that it is stripped off from the S-vlan tag and the customer’s own switch does the next step forwarding (based on the vlan tags).
I hope that was informative and will clear out a lot of common misunderstanding on QinQ and S-Vlans.