DELL N3000 User Manual

Configuring VLANs

For trunk ports, which are ports that connect one switch to another switch, 

the Dell Networking series switches support double-VLAN tagging. This 

feature allows service providers to create Virtual Metropolitan Area Networks 

(VMANs). With double-VLAN tagging, service providers can pass VLAN 

traffic from one customer domain to another through a metro core in a 

simple and cost-effective manner. By using an 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.