652 Dell PowerConnect 55xx Systems User GuideFILE LOCATION: C:\Users\gina\Desktop\Checkout_new\Maintenance Projects\DellContax\CxUGQoS.fmD E L L C O N F I D E N T I A L – P R E L I M I N A R Y 9 / 1 1 / 1 2 - F O R P R O O F O N LYQoS Features and ComponentsThe QoS feature is used to optimize network performance. It providesclassification of incoming traffic into traffic classes, based on one or moreattributes, including:– Device configuration– Ingress interface– Packet contentsQoS includes the following features:• Traffic Classification — Classifies each incoming packet, as belonging toa specific traffic flow, based on the packet contents and/or interface. Theclassification is done by an ACL (Access Control List), and only trafficthat meets the ACL criteria is subject to classification.• Assignment to Hardware Queues — Assigns incoming packets toforwarding queues. Packets are sent to a particular queue for handling as afunction of the traffic class to which they belong.• Other Traffic Class-Handling Attribute — Applies QoS mechanisms tovarious classes, including bandwidth management.QoS ModesA single QoS mode is selected and applies to all interfaces in the system. Themodes are:• Basic Mode — Class of Service (CoS).– Traffic is divided into classes that determine how it is treated. Alltraffic in a class is treated with the same QoS action. The QoS actionfor the class of traffic determines the egress queue on the egress port,based on the indicated QoS value in the incoming frame.The QoS value in the incoming frame is:• Layer 2 Packets — VLAN Priority Tag (VPT) 802.1p value• Layer 3 IPv4 Frames — Differentiated Service Code Point(DSCP) value• Layer 3 IPv6 Frames —Traffic Class (TC) valueWhen operating in Basic mode, the switch trusts thisexternally-assigned QoS value.
