Catalyst 2900 XL and 3500 XL series switches do not provide any priority scheduling in the input side. However, they do provide priority scheduling in the output/egress port.
Catalyst 2900 XL and 3500 XL series switches honor the incoming Class of Service (CoS) value in the dot1q tag. All the ports are considered trusted ports by default. Since dot1q does not tag the native VLAN traffic, issue the switchport priority default <0-7> interface level configuration command to classify the incoming untagged packet as desired. The switchport priority default <0-7> command is issued to provide CoS equivalent egress scheduling. If the egress port is a trunk port, the ingress CoS or port default priority configuration is marked in the outgoing frames as CoS values for the far-end device to treat them with the desired higher priority.
This URL should help you:
http://www.cisco.com/en/US/products/hw/switches/ps607/products_qanda_item09186a0080116ffe.shtml