Vlan Double Tagging (Vlan Stacking); How Double-Tagged Vlans Work; Vlan Rules For Double Tagging - Allied Telesis SwitchBlade x908 Series Software Reference Manual

VLAN Double Tagging (VLAN
Stacking)
VLAN double tagging, also known as VLAN Stacking, Nested VLANs, or Q-in-Q VLANs, are
used to operate a number of private Layer 2 networks within a single public Layer 2 network.
This feature provides simple access infrastructure for network service providers to operate
Metropolitan Area Networks (MANs) as commercial value added networks.
A nested VLAN implementation consists of the following port types:
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How double-tagged VLANs work

In a nested VLAN environment VLAN tagging exists at two levels:
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When nested VLAN functionality is enabled, the service provider assigns to each of its clients,
an individual 12 bit customer VID called an S-Tag. The S-Tag field has an identical structure to
a conventional VLAN tag field.
The S-Tag is attached to a packet as it enters the service provider network at the customer
edge port, and is removed as it leaves the destination customer edge port. From this point on,
the S-Tag is used for transmission within the service provider, or public Layer-2, network. The
VID that is used within the client's own network (now termed the C-Tag) is ignored by the
service provider network and bridging is based on the value of the S-Tag. The ethertype of the
S-Tag is set by changing the Tag Protocol Identifier (TPID).
Once the S-Tag is removed from the packet, it is forwarded "as is" out of the customer-edge
port. The tagged status of the Customer port is ignored on egress.

VLAN Rules for Double Tagging

When double-tagged VLANs are created on the switch:
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A service provider port:
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A customer edge port:
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C613-50007-01 REV B
Provider ports - these connect to a service provider's Layer-2 network
Customer edge ports - these connect to a customer's private Layer-2 network
client tagging,
service provider tagging.
a nested VLAN belongs to only one customer and can have multiple customer-edge ports
a port must be either a customer-edge port or a provider port, but cannot be both
accepts only tagged packets
transmits only tagged packets
can be in many double-tagged VLANs
accepts both tagged and untagged packets
transmits both tagged and untagged packets
can be a member of only one nested VLAN
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1
VLAN Introduction
16.5