Dell PowerConnect 5548 User Guide - Page 641

QoS Features and Components The QoS feature is used to optimize network performance. It provides classification of incoming traffic into traffic classes, based on one or more attributes, including: - Device configuration - Ingress interface - Packet contents QoS includes the following features: • Traffic Classification - Classifies each incoming packet, as belonging to a specific traffic flow, based on the packet contents and/or interface. The classification is done by an ACL (Access Control List), and only traffic that meets the ACL criteria is subject to classification. • Assignment to Hardware Queues - Assigns incoming packets to forwarding queues. Packets are sent to a particular queue for handling as a function of the traffic class to which they belong. • Other Traffic Class-Handling Attribute - Applies QoS mechanisms to various classes, including bandwidth management. QoS Modes A single QoS mode is selected and applies to all interfaces in the system. The modes are: • Basic Mode - Class of Service (CoS). - Traffic is divided into classes that determine how it is treated. All traffic in a class is treated with the same QoS action. The QoS action for 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) value When operating in Basic mode, the switch trusts this externally-assigned QoS value. Dell PowerConnect 55xx Systems User Guide 641