Cisco 3560G 48TS Software Configuration Guide - Page 551

Chapter 28 Configuring QoS Configuring Standard QoS Standard QoS Configuration Guidelines Before beginning the QoS configuration, you should be aware of this information: • You configure QoS only on physical ports; there is no support for it on the VLAN or switch virtual interface level. • It is not possible to match IP fragments against configured IP extended ACLs to enforce QoS. IP fragments are sent as best-effort. IP fragments are denoted by fields in the IP header. • Only one ACL per class map and only one match class-map configuration command per class map are supported. The ACL can have multiple ACEs, which match fields against the contents of the packet. • Incoming traffic is classified, policed, and marked down (if configured) regardless of whether the traffic is bridged, routed, or sent to the CPU. It is possible for bridged frames to be dropped or to have their DSCP and CoS values modified. • Only one policer is applied to a packet on an ingress port. Only the average rate and committed burst parameters are configurable. • The port ASIC device, which controls more than one physical port, supports 256 policers (255 policers plus 1 no policer). The maximum number of policers supported per port is 64. For example, you could configure 32 policers on a Gigabit Ethernet port and 8 policers on a Fast Ethernet port, or you could configure 64 policers on a Gigabit Ethernet port and 5 policers on a Fast Ethernet port. Policers are allocated on demand by the software and are constrained by the hardware and ASIC boundaries. You cannot reserve policers per port; there is no guarantee that a port will be assigned to any policer. • On a port configured for QoS, all traffic received through the port is classified, policed, and marked according to the policy map attached to the port. On a trunk port configured for QoS, traffic in all VLANs received through the port is classified, policed, and marked according to the policy map attached to the port. • You can create an aggregate policer that is shared by multiple traffic classes within the same policy map. However, you cannot use the aggregate policer across different policy maps. • If you have EtherChannel ports configured on your switch, you must configure QoS classification, policing, mapping, and queueing on the individual physical ports that comprise the EtherChannel. You must decide whether the QoS configuration should match on all ports in the EtherChannel. • Control traffic (such as spanning-tree bridge protocol data units [BPDUs] and routing update packets) received by the switch are subject to all ingress QoS processing. • You are likely to lose data when you change queue settings; therefore, try to make changes when traffic is at a minimum. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 28-29