Cisco 4500M Software Manual page 288

Understanding 802.1Q Tunneling
A port configured to support 802.1Q tunneling is called a tunnel port. When you configure tunneling,
you assign a tunnel port to a VLAN ID that is dedicated to tunneling. Each customer requires a separate
Service Provider VLAN ID, but that Service Provider VLAN ID supports VLANs of all the customers.
Customer traffic tagged in the normal way with appropriate VLAN IDs comes from an 802.1Q trunk port
on the customer device and into a tunnel port on the Service Provider edge switch. The link between the
customer device and the edge switch is asymmetric because one end is configured as an 802.1Q trunk
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. See
Figure 19-1 802.1Q 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 802.1Q-tagged with the appropriate VLAN ID. When the tagged packets exit the trunk port
into the Service Provider network, they are encapsulated with another layer of an 802.1Q tag (called the
metro tag) that contains the VLAN ID that is unique to the customer. The original customer 802.1Q tag
is preserved in the encapsulated packet. Therefore, packets entering the Service Provider network are
double-tagged, with the 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 metro
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.
Ethernet packets starting with the original, or normal, frame.
Software Configuration Guide—Release 12.2(25)EW
19-2
Customer A
VLANs 1 to 100
Tunnel port
VLAN 30
Tunnel port
VLAN 30
Tunnel port
VLAN 40
Customer B
VLANs 1 to 200
Chapter 19 Configuring 802.1Q and Layer 2 Protocol Tunneling
Figure 19-1.
Service
provider
Tunnel port
VLAN 30
Trunk Trunk
ports ports
Tunnel port
VLAN 40
Trunk
Asymmetric link
Figure 19-2
Customer A
VLANs 1 to 100
Customer B
VLANs 1 to 200
shows the tag structures of the
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