Modern networks are increasingly more complex, with new requirements being developed as new applications and ways of working appear. Sometimes there is a need for ISP customers to have their VLANs extended across the service provider network. The technique that supports this is called 802.1Q in Q, or more simply, 802.1Q tunneling. This is supported in the Catalyst 3550 series.
As you have read in this chapter, the 802.1Q protocol provides for a tag to be inserted inside the standard Ethernet frame carrying VLAN information. When 802.1Q tunneling is implemented, this happens twice. The first time, it is implemented by the customer, and the second time by the service provider.
At the tunnel boundary, the second tag, called the metro tag, is added, containing a VLAN ID unique to that customer. The frames are switched across the service provider network, and at the egress point, the metro tag is stripped and the exposed customer-specific 802.1Q frame is forwarded to the customer.
There are some restrictions to this technology, both in terms of the configuration options and the protocol operation. For example, these metro frames must be switched, not routed, and only layer 2 QoS is supported. Nonetheless, as more service providers offer switched networks across metropolitan areas, this is likely to become increasingly common.