Understanding Source Class Usage and Destination Class Usage Options
You can maintain packet counts based on the entry and exit points for traffic passing through your network. Entry and exit points are identified by source and destination prefixes grouped into disjoint sets defined as source classes and destination classes. You can define classes based on a variety of parameters, such as routing neighbors, autonomous systems, and route filters.
Source class usage (SCU) counts packets sent to customers by performing lookup on the IP source address and the IP destination address. SCU makes it possible to track traffic originating from specific prefixes on the provider core and destined for specific prefixes on the customer edge. You must enable SCU accounting on both the inbound and outbound physical interfaces.
Destination class usage (DCU) counts packets from customers by performing lookup of the IP destination address. DCU makes it possible to track traffic originating from the customer edge and destined for specific prefixes on the provider core router.
On T Series and M320 routers, the source class and destination classes are not carried across the platform fabric. The implications of this are as follows:
- On T Series and M320 routers, SCU and DCU accounting is performed before the packet enters the fabric.
- On T Series and M320 routers, DCU is performed before output filters are evaluated. On M Series platforms, DCU is performed after output filters are evaluated.
- If an output filter drops traffic on M Series platforms, the dropped packets are excluded from DCU statistics. If an output filter drops traffic on T Series and M320 routers, the dropped packets are included in DCU statistics.
Class-based filter match conditions are not supported on J Series Services Routers.
For more information about source class usage, see the Junos Policy Framework Configuration Guide, the Junos Network Interfaces Configuration Guide, and the Junos Feature Guide.