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This figure shows the tag structures of the double-tagged packets.

Figure 10: Original (Normal), IEEE 802.1Q, and Double-Tagged Ethernet Packet Formats

When the packet enters the trunk port of the service-provider egress switch, the outer tag is again stripped as

the switch internally processes the packet. However, the metro tag is not added when the packet is sent out

the tunnel port on the edge switch into the customer network. The packet is sent as a normal IEEE

802.1Q-tagged frame to preserve the original VLAN numbers in the customer network.

In the above network figure, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40.

Packets entering the edge switch tunnel ports with IEEE 802.1Q tags are double-tagged when they enter the

service-provider network, with the outer tag containing VLAN ID 30 or 40, appropriately, and the inner tag

containing the original VLAN number, for example, VLAN 100. Even if both Customers A and B have VLAN

100 in their networks, the traffic remains segregated within the service-provider network because the outer

tag is different. Each customer controls its own VLAN numbering space, which is independent of the VLAN

numbering space used by other customers and the VLAN numbering space used by the service-provider

network.

At the outbound tunnel port, the original VLAN numbers on the customer’s network are recovered. It is

possible to have multiple levels of tunneling and tagging, but the switch supports only one level in this release.

If traffic coming from a customer network is not tagged (native VLAN frames), these packets are bridged or

routed as normal packets. All packets entering the service-provider network through a tunnel port on an edge

switch are treated as untagged packets, whether they are untagged or already tagged with IEEE 802.1Q headers.

The packets are encapsulated with the metro tag VLAN ID (set to the access VLAN of the tunnel port) when

they are sent through the service-provider network on an IEEE 802.1Q trunk port. The priority field on the

metro tag is set to the interface class of service (CoS) priority configured on the tunnel port. (The default is

zero if none is configured.)

On switches, because 802.1Q tunneling is configured on a per-port basis, it does not matter whether the switch

is a standalone switch or a stack member. All configuration is done on the stack master.