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6.3.7 Q-in-Q VLAN (VLAN Stacking)
Q-in-Q tunneling is also known as VLAN stacking. Both of them use 802.1q double tagging
technology. Q-in-Q is required by ISPs (Internet Service Provider) that need Transparent LAN
services (TLS), and the service provider has their own set of VLAN, independent of customer
VLANs. Typically, each service provider VLAN interconnects a group of sites belonging to a
customer. However, a service provider VLAN could also be shared by a set of customers sharing
the same end points and quality of service requirements of the VLAN. Double tagging is
considered to be a relatively simpler way of implementing transparent LAN. This is
accomplished by encapsulating Ethernet Frame. A second or outer VLAN tag is inserted in
Ethernet frames sent over the ingress PE (Provider Edge). This VLAN tag corresponds to the
VLAN of the Service Provider (SP). When the frame reaches the destination PE, the SP VLAN
is stripped off. The DA of the encapsulated frame and the VLAN ID are used to take further L2
decisions, similar to an Ethernet frame arriving from a physical Ethernet port. The SP VLAN tag
determines the VPLS (Virtual Private LAN Service) membership. Double tagging aggregates
multiple VLANs within another VLAN and provides a private, dedicated Ethernet connection
between customers to reach their subnet transparently across multiple networks. Thus service
providers can create their own VLANs without interfering with customer VLANs by using
double tagging. This allows them to connect customers to ISPs and ASPs (Application Service
Provider).
The ports that are connected to the service provider VLANs are called tunnel ports, and the ports
that are connected to the customer VLANs are called access (subscriber/customer) ports. When
a port is configured as tunnel port, all the outgoing packets on this port will be sent out with
SPVLAN (SPVID and 1p priority) tag. The incoming packet can have two tags (SPVLAN +
CVLAN), one tag (SPVLAN or CVLAN), or no tag. In all cases, the packet is sent out with a
SPVLAN tag. When a port is configured as an access port, the incoming traffic can have only a
CVLAN (CVID and 1p priority) tag or no tag. Hence, all the packets that are being sent out of
access ports will be untagged or single tagged (CVLAN). When a port is configured as a normal
port, it will ignore the frames with double tagging.
Double Tagging Format
A VLAN tag (service provider VLAN stacking or customer IEEE 802.1Q) consists of the
following three fields.
TPID
Priority
VID
TPID (Tag Protocol Identifier) is a standard Ethernet type code identifying the frame and
indicates that whether the frame carries IEEE 802.1Q tag information. The value of this field is
0x8100 as defined in IEEE 802.1Q. Other vendors may use a different value, such as 0x9100.
Tunnel TPID is the VLAN stacking tag type the Switch adds to the outgoing frames sent through
a Tunnel Port of the service provider's edge devices
Priority refers to the IEEE 802.1p standard that allows the service provider to prioritize traffic
based on the class of service (CoS) the customer has paid for. "0" is the lowest priority level and
"7" is the highest.
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