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port, and the other end is configured as a tunnel port. You assign the tunnel port interface to an access VLAN
ID that is unique to each customer.
Figure 9: IEEE 802.1Q Tunnel Ports in a Service-Provider Network
Packets coming from the customer trunk port into the tunnel port on the service-provider edge switch are
normally IEEE 802.1Q-tagged with the appropriate VLAN ID. The tagged packets remain intact inside the
switch and when they exit the trunk port into the service-provider network, they are encapsulated with another
layer of an IEEE 802.1Q tag (called the metro tag) that contains the VLAN ID that is unique to the customer.
The original customer IEEE 802.1Q tag is preserved in the encapsulated packet. Therefore, packets entering
the service-provider network are double-tagged, with the outer (metro) tag containing the customer’s access
VLAN ID, and the inner VLAN ID being that of the incoming traffic.
When the double-tagged packet enters another trunk port in a service-provider core switch, the outer tag is
stripped as the switch processes the packet. When the packet exits another trunk port on the same core switch,
the same metro tag is again added to the packet.
Catalyst 2960-XR Switch VLAN Configuration Guide, Cisco IOS Release 15.0(2)EX1
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Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
IEEE 802.1Q Tunneling