Cisco 3845 Software Guide - Page 499

Chapter 27 Configuring QoS Understanding QoS To communicate an ACL classification to an output policy, you assign a QoS number to specify packets at ingress. This example identifies specific packets as part of QoS group 1 for later processing in an output policy: Switch(config)# policy-map in-gold-policy Switch(config-pmap)# class in-class1 Switch(config-pmap-c)# set qos-group 1 Switch(config-cmap-c)# exit Switch(config-cmap)# exit You use the set qos-group command only in an input policy. The assigned QoS group identification is then used in an output policy with no mark or change to the packet. You use the match qos-group in the output policy. You cannot configure match qos-group for an input policy map. This example creates an output policy to match the QoS group created in the input policy map in-gold-policy. Traffic internally tagged as qos-group 1 is identified and processed by the output policy. Switch(config)# class-map out-class1 Switch(config-cmap)# match qos-group 1 Switch(config-cmap)# exit The switch supports a maximum of 100 QoS groups. Classification Based on Discard Class A discard class is very similar to a QoS group in that it is a virtual packet marking that is carried in the packet within a single device. The difference is that a QoS group defines the complete QoS behavior for a packet, while a discard class only communicates the drop precedence of the packet during congestion management. For example, a packet could be classified on input into one QoS group, but within that QoS group, a policer could mark one of three discard classes, depending on whether the packet was determined to conform to, exceed, or violate the configured specifications. On output, a class would match the QoS group. but you could configure three different drop curves, one for each of the discard classes. The discard class value ranges from 0 to 7. Classification Based on VLAN IDs With classification based on VLAN IDs, you can apply QoS policies to frames carried on a user-specified VLAN for a given interface. You can use hierarchical policy maps for per-VLAN classification on trunk ports. Per-VLAN classification is not required on access ports because access ports carry traffic for a single VLAN. You use the match vlan vlan-id class-map configuration command to classify based on the outer VLAN. Use the match vlan inner vlan-id class-map configuration command to classify based on the inner VLAN Classification for MPLS and EoMPLS In an MPLS network, QoS can be specified in different ways. For example, the IP precedence field (the first 3 bits of the DSCP field in the header of an IP packet) can specify the QoS value to give the packet. If the network is an MPLS network, the IP precedence bits are copied into the MPLS EXP field at the edge of the network. If a service provider wants to set a QoS value for an MPLS packet to a different value, instead of overwriting the value in the IP precedence field that belongs to a customer, the service provider can set the MPLS experimental field. The IP header remains available for the customer's use, and the QoS of an IP packet is not changed as the packet travels through the MPLS network. OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 27-11