Suppose we are a service provider carrying multiple tenants across our network. This is actually pretty simple and doesn’t require carrier-grade equipment.
In this example we’ll have an inner tag (customer) and an outer tag (provider). We’ll add 14 to the max mtu size on the provider switch to accommodate for the second tag.
The outer tag uses both the “switchport mode access” to define the customer and “switchport mode dot1q-tunnel” to tell that it’s twice encapsulated.
You can “hop off” the outer tag at the CPE depending on what vlan you assign them.
This is the basics of q-in-q. With this you can provide “metro-e” or “private clouds” really its just basic vlans inside of vlans!
So can you also do router-on-a-router-on-a-stick ? Sure can..
2921(config)# interface gigabitethernet 0/1/0 2921(config-if)# dot1q tunneling ethertype 0x9100 2921(config-if)# interface gigabitethernet 1/1/0.1 2921(config-subif)# encapsulation dot1q 100 second-dot1q 200
Read more about it : at cisco’s site