http://www.cisco.com/en/US/partner/products/hw/switches/ps607/products_qanda_item09186a0080116ffe.shtml
Q. What kind of output scheduling do the Catalyst 2900 XL and 3500 XL series switches provide?
A. Catalyst 2900 XL and 3500 XL series switches support two queues per port on the 10/100 and Gigabit Ethernet interfaces. Port priority value or Class of Service (COs) value 0-3 maps to a low priority queue on the egress port and port priority value or COs value 4-7 maps to the high priority queue on the egress port(s).
COs/Port Priority Queue Selected
0-3 Q1 (Lower priority)
4-7 Q2 (Higher priority)
Priority scheduling is applied between the queues, which will assure that the high priority queue is always serviced before scheduling the low priority traffic. These features enable us to prioritize mission critical traffic, such as IP telephony, over regular traffic, such as File Transfer Protocol (FTP) or generic Web. The low priority queues will experience tail drop during congestion when there is traffic in the high priority queue.
