Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 405
53-1002269-02
Chapter
15
Configuring Quality of Service
Classification
Quality of Service (QoS) features are used to prioritize the use of bandwidth in a switch. When QoS
features are enabled, traffic is classified as it arrives at the switch, and processed through on the
basis of configured priorities. Traffic can be dropped, prioritized for guaranteed delivery, or subject
to limited delivery options as configured by a number of different mechanisms.
This chapter describes how QoS is implemented and configured in PowerConnect devices.
Classification is the process of selecting packets on which to perform QoS, reading the QoS
information and assigning a priority to the packets. The classification process assigns a priority to
packets as they enter the switch. These priorities can be determined on the basis of information
contained within the packet or assigned to the packet as it arrives at the switch. Once a packet or
traffic flow is classified, it is mapped to a forwarding priority queue.
Packets on devices are classified in up to eight traffic classes with values between 0 and 7. Packets
with higher priority classifications are given a precedence for forwarding.
Processing of classified traffic
The trust level in effect on an interface determines the type of QoS information the device uses for
performing QoS. The device establishes the trust level based on the configuration of various
features and if the traffic is switched or routed. The trust level can be one of the following:
• Ingress port default priority
• Static MAC address
• Layer 2 Class of Service (CoS) value – This is the 802.1p priority value in the Ethernet frame. It
can be a value from 0 – 7. The 802.1p priority is also called the Class of Service.
• Layer 3 Differentiated Service codepoint (DSCP) – This is the value in the six most significant
bits of the IP packet header 8-bit DSCP field. It can be a value from 0 – 63. These values are
described in RFCs 2472 and 2475. The DSCP value is sometimes called the DiffServ value.
The device automatically maps a packet's DSCP value to a hardware forwarding queue. Refer
to “Viewing QoS settings” on page 418".
• ACL keyword – An ACL can also prioritize traffic and mark it before sending it along to the next
hop. This is described in the ACL chapter in the section “QoS options for IP ACLs” on page 387.
Given the variety of different criteria, there are multiple possibilities for traffic classification within a
stream of network traffic. For this reason, the priority of packets must be resolved based on which
criteria takes precedence. Precedence follows the scheme illustrated in Figure 82
Determining the trust level of a packet’
Figure 82 illustrates how the device determines the trust level of a packet.
FIGURE 82 Determining a packet trust level