Overview; Single-Rate And Twin-Rate Policer - Allied Telesis AT-8100L/8 User Manual

Chapter 100: Quality of Service (QoS)

Overview

Single-rate and
Twin-rate Policer
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Quality of Service (QoS) refers to the latency, bandwidth, jitter, and loss
settings that high-bandwidth traffic, such as voice traffic, require to
maintain traffic quality. As more companies convert their phone systems to
VoIP, QoS becomes even more important because this feature allows you
to prioritize traffic to ensure voice quality. By default, QoS is disabled on
the switch.
At the core of the QoS is the policy map. The policy map contains one or
more class maps that filter traffic based on matching criterion, including
Cost of Service (CoS), Diffserv Code Point (DSCP), VLAN ID, or MAC
address. DSCP is used as a match criterion for Layer 3 packets while Cost
of Service (CoS) is used as a match criterion for Layer 2 frames. You can
add multiple class maps, typically each with unique matching criterion, to
one policy map. It is important to note that class map settings apply to
ingress traffic only.
A policy map allows you to set actions on traffic that meet all of the match
criterion contained in the class maps. In other words, once you have
defined the traffic that you want to filter, you decide what you want to do
with that traffic. There are three choices, you can permit the specified
traffic, you can deny the specified traffic, or you can monitor the specified
traffic by copying it to a port mirror. The classified traffic in a policy map is
denied by default. After you have added the desired class maps to a policy
map, you associate the policy map with an interface to make the filter
active.
In addition, a default class map is provided to save time when configuring
QoS. A default class map enables you to specify the action that applies to
all unclassified traffic within a policy map. You can choose from permit,
deny, or copy to a mirror port.
Once you create a policy map populated with one or more class maps,
you can apply other QoS settings to traffic, as described in the following
sections.
There are two types of policers available in QoS, single-rate and twin-rate.
If traffic does not conform to the conditions set in a policer command, both
the single-rate and twin-rate policer can either drop or remark traffic. A
single-rate policer allows you to determine the following:
 Committed Information Rate (CIR)

Committed burst size (CBS)

Excess Burst Size (EBS)