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 lookups 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 lookups 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 Core Routers and M320 Multiservice Edge 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 devices, 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.
For PTX Series routers with FPC3, and PTX1000 routers, to support SCU and DCU, you must configure enhanced-mode on the chassis.
On MX Series platforms with MPC/MIC interfaces, SCU and DCU are performed after output filters are evaluated. Packets dropped by output filters are not included in SCU or DCU statistics.
On MX Series platforms with non-MPC/MIC interfaces, SCU and DCU are performed before output filters are evaluated. Packets dropped by output filters are included in SCU and DCU statistics.
On PTX Series platforms, SCU and DCU accounting is performed before output filters are evaluated. Packets dropped by output filters are included in SCU and DCU statistics. On PTX10003, PTX10004, PTX10008, PTX10001-36MR, and line card JNP10K-LC1201, the systems prefixes with SCU and DCU classes assigned occupy more space in the forwarding information base (FIB) tables than regular routes. You must limit the number of prefixes with non-default class assigned.
On Enhanced Scaling FPCs (T640-FPC1-ES, T640-FPC2-ES, T640-FPC3-ES, T640-FPC4-1P-ES , and T1600-FPC4-ES), the source class accounting is performed at ingress. Starting with Junos OS Release 14.2, the SCU accounting is performed at ingress on a T4000 Type 5 FPC. The implications of this are as follows:
SCU accounting is performed when packets traverse from T4000 Type 5 FPC (ingress FPC) to Enhanced Scaling FPCs (egress FPC).
SCU accounting is performed when packets traverse from Enhanced Scaling FPCs (ingress FPC) to T4000 Type 5 FPC (egress FPC).
When the interface statistics are cleared and then the routing engine is replaced, the SCU and DCU statistics will not match the statistics of the previous routing engine.
For more information about source class usage, see the Routing Policies, Firewall Filters, and Traffic Policers User Guide and the Junos OS Network Interfaces Library for Routing Devices.