Configuring Quality of Service 687Class of ServiceThe Class of Service (CoS) queueing feature lets you directly configure certain aspects of switchqueueing. This provides the desired QoS behavior for different types of network traffic when thecomplexities of DiffServ are not required. The priority of a packet arriving at an interface can be used tosteer the packet to the appropriate outbound CoS queue through a mapping table. CoS queuecharacteristics that affect queue mapping, such as minimum guaranteed bandwidth, transmission rateshaping, etc., are user-configurable at the queue (or port) level.Seven queues per port are supported. Although the hardware supports eight queues, one queue is alwaysreserved for internal use by the stacking subsystem.To display the page, click Quality of Service →Class of Service in the tree view. The Class of Servicemenu page contains links to the following features:• Mapping Table Configuration• Interface Configuration• Interface Queue ConfigurationMapping Table ConfigurationEach port in the switch can be configured to trust one of the packet fields (802.1p, IP Precedence, or IPDSCP), or to not trust any packet’s priority designation (untrusted mode). If the port is set to a trustedmode, it uses a mapping table appropriate for the trusted field being used. This mapping table indicatesthe CoS queue to which the packet should be forwarded on the appropriate egress port(s). Of course, thetrusted field must exist in the packet for the mapping table to be of any use, so there are default actionsperformed when this is not the case. These actions involve directing the packet to a specific CoS levelconfigured for the ingress port as a whole, based on the existing port default priority as mapped to atraffic class by the current 802.1p mapping table.Alternatively, when a port is configured as untrusted, it does not trust any incoming packet prioritydesignation and uses the port default priority value instead. All packets arriving at the ingress of anuntrusted port are directed to a specific CoS queue on the appropriate egress port(s), in accordance withthe configured default priority of the ingress port. This process is also used for cases where a trusted portmapping is unable to be honored, such as when a non-IP packet arrives at a port configured to trust theIP DSCP value.Use the Mapping Table Configuration page to define how class of service is assigned to a packet.To display the page, click Quality of Service →Class of Service →Mapping Table Configuration in thetree view.Table 11-7. DiffServ Statistics CommandsCLI Command Descriptionshow diffserv service brief Displays all interfaces in the system to which a DiffServ policy hasbeen attached.
