Dell PowerConnect M6220, User Configuration Manual

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Configuring VLANs
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additional tag on the traffic, the switch can differentiate between customers
in the MAN while preserving an individual customer’s VLAN identification
when the traffic enters the customer’s 802.1Q domain.
With the introduction of this second tag, customers are no longer required to
divide the 4-byte VLAN ID space to send traffic on a Ethernet-based MAN.
In short, every frame that is transmitted from an interface has a double-VLAN
tag attached, while every packet that is received from an interface has a tag
removed (if one or more tags are present).
In Figure 21-2, two customers share the same metro core. The service
provider assigns each customer a unique ID so that the provider can
distinguish between the two customers and apply different rules to each.
When the configurable EtherType is assigned to something different than the
802.1Q (0x8100) EtherType, it allows the traffic to have added security from
misconfiguration while exiting the metro core. For example, if the edge
device on the other side of the metro core is not stripping the second tag, the
packet would never be classified as a 802.1Q tag, so the packet would be
dropped rather than forwarded in the incorrect VLAN.