VLAN IN BROADBAND

How vlan is used in dslam and broadband. What is Q-in-Q.
Let us taken an example.
A broadband user is terminated at port no.1 of dslam no.1.

The dslam allots vlan id.128 to this user.

The dslam has to aggrgate all its users and send it to the next stage in a single set of wires. So it put a tag known 802.1Q.

The next stage receives these frames. But the next stage has to aggregate such kind of stream from other dslams also.What I mean is, there can be other frames from other dslams having the same vlan id :128. But its duty is , he has aggreage all the frames and send it next stage. Here, the protocol 802.1Q does not allow putting another vlan tag. So the switch uses another slightly different protocol which uses two tags. its name is 802.1QinQ. Or simply called as Q-in-Q. It adds another vlan tag (say 3000) which is the identity for this switch. Now our user gets a unique id 3000-128.

With two tags, the frame enters into router called as BRAS, Now the bras knows who is the owner of this frame. Note: This is a very rough sketch of what happens to your ip packet in broadband. It need not be correct.