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6.3.4.
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 ( 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.