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Software Configuration Guide—Release 12.2(25)SG
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Chapter18 Configuring 802.1Q and Layer 2 Protocol Tunneling
Understanding 802.1Q Tunneling
A port configured to support 802.1Q tunneling is called a tunne l port. When you configure tunneling,
you assign a tunnel port to a VLAN ID that is ded icated to tunneling. Each customer requires a separate
Service Provider VLAN ID, but that Service Provider VLANID supports VLANs of all the customers.
Customer traffic tagged in the normal way with appropriate VLAN IDs comes from an 802.1Q trunk p ort
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 Figure18-1.
Figure18-1 802.1Q 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 pa ckets exit the trunk port
into the Service Provider network, they are encapsulated w ith 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 metr o
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. Figure18-2 shows the tag structures of the
Ethernet packets starting with the original, or normal, frame.
Customer A
VLANs 1 to 100
Customer B
VLANs 1 to 200
Customer B
VLANs 1 to 200
Customer A
VLANs 1 to 100
Tunnel port
VLAN 40
Tunnel port
VLAN 30
Trunk
ports
Trunk
ports
Tunnel port
VLAN 30
Tunnel port
VLAN 40
Service
provider
74016
Trunk
Asymmetric link
Tunnel port
VLAN 30