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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
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
frame
DA SA Len or
Etype
Data FCS
single-
tagged
frame
DA SA TPID
P
VID
Len or
Etype
Dat
a
FCS
double-
DA SA Tunne
P
VID
TPID
P
VID
Len or
Dat
FCS
