Specifications

Configuring Routing Between VLANs
How to Configure Routing Between VLANS
42
The Q-in-Q VLAN tag termination feature is simpler than the IEEE 802.1Q tunneling feature deployed
for the Catalyst 6500 series switches or the Catalyst 3550 and Catalyst 3750 switches. Whereas switches
require IEEE 802.1Q tunnels on interfaces to carry double-tagged traffic, routers need only encapsulate
Q-in-Q VLAN tags within another level of 802.1Q tags in order for the packets to arrive at the correct
destination as shown in Figure 81.
Figure 81 Untagged, 802.1Q-Tagged, and Double-Tagged Ethernet Frames
Cisco 10000 Series Internet Router Application
For the emerging broadband Ethernet-based DSLAM market, the Cisco 10000 series Internet router
supports Q-in-Q encapsulation. With the Ethernet-based DSLAM model shown in Figure 82, customers
typically get their own VLAN and all these VLANs are aggregated on a DSLAM.
Double-tag
g
frame
802.1Q frame fro
m
customer network
Original Ethernet frame
Destination
address
Length/
EtherType
Frame Check
Sequence
Source
address
SADA Len/Etype Data FCS
SADA Len/Etype DataEtype Ta g FCS
SADA Len/Etype DataEtype Ta g Etype Ta g FCS