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Source Class Usage Overview

Source class usage (SCU) is a logical extension of the destination class usage (DCU) concept. DCU was created so that Juniper Networks customers could count on a per-interface basis how much traffic was sent to specified prefixes. Figure 1 shows a service provider edge (PE) router diagram.

Figure 1: DCU/SCU ConceptDCU/SCU Concept

The Fast Ethernet interfaces contain inbound traffic from customers, and the SONET/SDH interfaces are connected to outbound public network prefixes. With DCU configured on the Fast Ethernet interfaces, you can track how much traffic is sent to a specific prefix in the core of the network originating from one of the specified interfaces (in this case, the Fast Ethernet interfaces).

However, DCU limits your ability to keep track of traffic moving in the reverse direction. It can account for all traffic that arrives on a core interface and heads toward a specific customer, but it cannot count traffic that arrives on a core interface from a specific prefix. For example, DCU can process cumulative traffic headed toward interface fe-0/0/0, but cannot differentiate between traffic coming only from and traffic coming from all prefixes.

You can track source-based traffic by using SCU, which allows you to monitor the amount of traffic originating from a specific prefix. With this feature, usage can be tracked and customers can be billed for the traffic they receive.