C H A P T E R 17 Configuring Ieee 802.1Q And Layer 2 Protocol Tunneling - Cisco WS-CBS3032-DEL Software Configuration Manual

Understanding IEEE 802.1Q Tunneling
Customer traffic tagged in the normal way with appropriate VLAN IDs comes from an IEEE 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
IEEE 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 17-1
802.1Q trunk port
802.1Q trunk port
802.1Q trunk port
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 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.
double-tagged packets.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
17-2
IEEE 802.1Q Tunnel Ports in a Service-Provider Network
Customer A
VLANs 1 to 100
802.1Q trunk port
802.1Q trunk port
Tunnel port
VLAN 30
Tunnel port
VLAN 30
Tunnel port
VLAN 40
Customer B
VLANs 1 to 200
Chapter 17 Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
Service
provider
Tunnel port
VLAN 30
Trunk Trunk
ports ports
Tunnel port
VLAN 40
Trunk
Asymmetric link
Figure 17-2
Figure 17-1.
Customer A
VLANs 1 to 100
802.1Q trunk port
802.1Q trunk port
802.1Q trunk port 802.1Q trunk port
Customer B
VLANs 1 to 200
shows the tag structures of the
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