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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
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
frame
single-tagged
DA SA
frame
double-tagged
DA SA
frame
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
Data: Frame data
FCS: Frame Check Sequence
VLAN Stacking Port Roles
Each port can have three VLAN stacking "roles", Normal, Access Port and Tunnel Port.
Select Normal for "regular" (non-VLAN stacking) IEEE 802.1Q frame switching.
Select Access Port for ingress ports on the service provider's edge devices. The incoming
Priority
VID
Len or
Data
FCS
Etype
TPID
P
VID
Tunnel
P
VID
TPID
120
Len or
Data
FCS
Etype
TPID
P
VID
Len or
Data
FCS
Etype
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