Q-In-Q Vlan (Vlan Stacking) - TRENDnet TI-PG1284i User Manual

TRENDnet User's Guide

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
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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.
VID is the VLAN ID. SP VID is the VID for the second or outer (service provider's) VLAN tag.
CVID is the VID for the first or inner (Customer's) VLAN tag.
The frame formats for an untagged Ethernet frame; a single-tagged 802.1Q frame
(customer) and a "double-tagged" 802.1Q frame (service provider) are shown as
following.
untagged DA SA Len or
frame Etype
single-tagged DA SA TPID
frame
double-tagged DA SA Tunnel
frame TPID
DA: Destination Address
SA: Source Address
Tunnel TPID: Tag Protocol Identifier added on a tunnel port
P: 802.1p priority
VID: VLAN ID
Len or Etype: Length or Ethernet frame type
TI-PG1284i
Data FCS
P VID Len or Data FCS
Etype
P VID TPID P VID Le
Et
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