12 802.1Q Tunneling
12.1 Understanding 802.1Q Tunneling
The commercial users of the network service providers usually have special requirements
for the supported VLAN and VLAN IDs. There may be superposition in the range of the
VLANs needed by the users of the same vendor, and the switching channels of different
users through the core network of the vendors may be mixed together. To define a VLAN
range for every individual user may cause restrictions on the user configurations, and the
VLAN number of 4096, as defined by the 802.1Q, may be easily exceeded.
The features of the IEEE 802.1Q Tunneling enable the vendor to use one VLAN (vendor
VLAN) to support the users with multiple VLANs. The VLAN of the user is reserved. In this
way, the traffic of different users to the vendor can be transmitted separately in the vendor's
internal network even if its VLANs are the same. Through dual Tags, the tunneling extends
the range of the VLAN. A port that supports the IEEE 802.1Q Tunneling is called a tunnel
port. In the configuration of tunneling, a VLAN can be assigned to the tunnel port as the
dedicated VLAN. Thus, every user just needs to use the VLAN of one vendor. The user's
traffic is packaged into dual-tagged frames while being transmitted in the vendor's network,
and is transmitted in the network through the VLAN of the vendor.
The switching traffic of the user goes from one of its TRUNK port, carrying normal 802.1Q
TAGs, to a tunnel port of the edge device of the vendor. Such an asymmetrical connection
between the user and vendor is called the asymmetrical link, because one end is to a Trunk
port while the other end to a tunnel port. The tunnel ports of different users are assigned with
different VLANs. See the following application scheme diagram:
Figure 12-1
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