Quality of Service

This chapter discusses the Quality of Service (QoS) feature, and allows you to configure a switch to provide different levels of service to different groups of traffic. In this section you will find both overview information, as well as specific information on how to configure and monitor the QoS feature.

QoS Overview

Quality of Service (QoS) is a feature that allows you to configure a switch to provide different levels of service to different groups of traffic. For example, QoS allows you to do the following:

Give some traffic groups higher priority access to network resources.
Reserve bandwidth for special traffic groups.
Restrict some traffic groups to bandwidth or data rates defined in a Service Level Agreement (SLA).
Count frames and packets that exceed specified limits and optionally discard them (rate limiting).
Queue or buffer frames and packets that exceed specified limits and forward them later (rate shaping).
Modify QoS related fields in forwarded frames and packets (remarking).

QoS on Extreme Networks Switches shows the QoS components that provide these features on Extreme Networks switches.

QoS on Extreme Networks Switches

Data enters the ingress port and is sorted into traffic groups, which can be classified as either ACL-based or non-ACL-based.

The ACL-based traffic groups provide the most control of QoS features and can be used to apply ingress and egress rate limiting and rate shaping as follows:

Subject ingress traffic to rate limit meters
Specify ingress hardware queues (QoS profiles) for rate limiting and rate shaping
Specify ingress software traffic queues for rate limiting and rate shaping (these can be associated with egress traffic queues for additional QoS control)
Specify egress software traffic queues for rate limiting and rate shaping
Specify egress QoS profiles for rate limiting and rate shaping
Change the dot1p or Differential Services (DiffServ) values in egress frames or packets

Non-ACL-based traffic groups specify an ingress or egress QoS profile for rate limiting and rate shaping. These groups cannot use ingress or egress software traffic queues. However, non-ACL-based traffic groups can use the packet marking feature to change the dot1p or DiffServ values in egress frames or packets.

The ingress rate-limiting and rate-shaping features allow you to apply QoS to incoming traffic before it reaches the switch fabric. If some out-of-profile traffic needs to be dropped, it is better to drop it before it consumes resources in the switch fabric.

All ingress traffic is linked to an egress traffic queue or QoS profile before it reaches the switch fabric. This information is forwarded with the traffic to the egress interface, where it selects the appropriate egress traffic queue or QoS profile. Egress traffic from all traffic queues and QoS profiles is forwarded to the egress port rate-shaping feature, which applies QoS to the entire port. When multiple QoS profiles are contending for egress bandwidth, the scheduler determines which queues are serviced.