How does ATM-layer traffic shaping work on the PA-A3-OC12?
A. The PA-A3-OC12 supports VBR-nrt and UBR ATM service categories. The SAR assigns a lower internal priority value to UBR virtual channels (VCs) than VBR-nrt VCs. The SAR first schedules a cell from a VBR-nrt VC in an available cell timeslot. If no VBR VCs have data for transmission or if the interface is supporting a single VC, then the entire link bandwidth is available for the UBR VC. Unlike the PA-A3-OC3 or PA-A3-DS3, the PA-A3-OC12 does not support the transmit priority command to manually change the priority level of a VC. When two VBR-nrt VCs compete for the same cell timeslot, the SAR reschedules one VC for a later cell timeslot. To ensure that such rescheduling does not lead to reduced throughput, the PA-A3-OC12 implements a bandwidth recovery algorithm in the SAR microcode v3.2. For more information, refer to the Understanding Traffic Shaping Accuracy section of Traffic Shaping on ATM Line Cards for the Cisco 12000 Series. When no VBR-nrt VC needs to use a cell timeslot, the
