Figure 36 Ethernet in Multicustomer Environments

Carrier-Owned Switch

CEp^ Customer 1

I "ir

Customer 2

Customer 3

Switch

Elhernel

Riser

Although id entifying th e customer i tsele is easy, isolating tee traffic between t he di fferent customers becomes an interesting issue and requires some attention on the provider's part. Without special attention, hra ffic might get exch anged between th e different custom ers in the bu ilding through the baseme nt switch1 You have already seen in the section "L2 Switc hing .Basic^' that VLANs can be used to separate physi cal seg ments in to m an y logical segments; however, thus wovks in a single-customes environment, where the VLAN has a global meaning. In e hnulticustohaer enviro nme nt, each cestomer can have its own set of VLANs that overlap with VLANs from another customer. To work in this environment, carriers are adopting a model very similar to how Frame Relay and ATM services have been deployed. In essence, each customer is given service identifiers similar to Frame Relay data-link connection identifiers (DLCIs), which identify EVCs over which the customer's traffic travels. In the case of Ethernet, the VLAN ID given by a carrier becomes that identifier. This is illustrated in Figure 3-7.

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