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How Can I Implement Quality of Service?
required, packet loss, or availability requirements of the flow. QoS provides management
mechanisms for these flow characteristics.
QoS achieves its bandwidth management capabilities by:
•
Setting priorities that define traffic handling
•
Dedicating bandwidth and prioritizing queuing for specific applications, and reducing packet
transmission delay and jitter
•
Managing congestion by shifting packet loss to applications that can tolerate it
How Can I Implement Quality of Service?
QoS determines how a flow will be treated as it transits the link. To determine how a flow should
be treated, you must first understand the characteristics of the flows on your network, and
secondly, you must identify these flows in a way that QoS can recognize. In this sense, QoS is the
third step in a three step process. The three‐steps Enterasys recommends for configuring QoS are:
•
Understand your network flows using NetFlow
•
Associate the flows on your network with a well defined role using Enterasys policy
•
Configure the appropriate link behavior for that role by associating the role with a QoS
configuration
Quality of Service Overview
QoS is all about managing the bandwidth in a manner that aligns the delivery characteristics of a
given flow with the available port resources. In a QoS context, a flow is a stream of IP packets that
are classified with the same class of service as it transits the interface. QoS manages bandwidth for
each flow by taking advantage of its ability to:
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Assign different priority levels to different packet flows
•
Mark or re‐mark the packet priority at port ingress with a Type of Service
•
Sort flows by transit queue such that a higher priority queue gets preferential access to
bandwidth during packet forwarding
•
Limit the amount of bandwidth available to a given flow by either dropping (rate limit) or
buffering (rate shape) packets in excess of configured limits
These QoS abilities collectively make up a Class of Service (CoS). The remainder of this section
will briefly describe CoS and its components.
Class of Service (CoS)
You implement QoS features in a Class of Service (CoS). How the firmware treats a packet as it
transits the link depends upon the priority and forwarding treatments configured in the CoS. Up
to 256 unique CoS entries can be configured. CoS entries 0–7 are configured by default with an
802.1p priority assigned and default forwarding treatment. For purposes of backward
compatibility, CoS entries 0–7 cannot be removed. CoS entries 8‐255 can be configured for the
following services:
•
802.1p priority
•
IP ToS rewrite value
February 22, 2008
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