802.1Q Tunneling

802.1Q Tunneling
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.
478 Chapter 14  VLANs, Trunks, and VTP
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 it is implemented 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.
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