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Section 12. Conguring Quality of Service
This section contains the following topics:
Quality of Service Overview
Enabling Quality of Service
Mapping Queues
12.1 Quality of Service Overview
Network trafc is usually unpredictable, and the only basic assurance that can be offered is best effort trafc delivery. To
overcome this challenge, Quality of Service (QoS) is applied throughout the network. This ensures that network trafc is
prioritized according to specied criteria, and that specic trafc receives preferential treatment. QoS in the network optimizes
network performance and entails two basic facilities:
Classifying incoming trafc into handling classes, based on an attribute, including:
– The ingress interface
– Packet content
– A combination of these attributes
Providing various mechanisms for determining the allocation of network resources to different handling classes, including:
– The assignment of network trafc to a particular hardware queue
– The assignment of internal resources
– Trafc shaping
In this document, the terms Class of Service (CoS) and QoS are used in the following context:
CoS provides varying Layer 2 trafc services. CoS refers to classication of trafc to trafc-classes, which are handled as
an aggregate whole, with no per-ow settings. CoS is usually related to the 802.1p service that classies ows according
to their Layer 2 priority, as set in the VLAN header.
QoS refers to Layer 2 trafc and above. QoS handles per-ow settings, even within a single trafc class.
The QoS facility involves the following elements:
Trafc Classication — Classies each incoming packet as belonging to a given trafc class, based on the packet
contents and/or the context.
Assignment to Hardware Queues — Assigns incoming packets to forwarding queues. Packets are sent to a particular
queue for handling as a function of the trafc class to which they belong, as dened by the classication mechanism.
Trafc Class-Handling Attributes — Applies QoS/CoS mechanisms to different classes, including:
– Bandwidth Management
– Shaping/ Rate Limiting
– Policing
12.1.1 Mapping to Queues
Queues are used in both Basic and Advanced QoS modes. Default settings are applied to maps in Service QoS mode. A
Trust Behavior can be selected, or the output service elds can be selected, including:
VLAN Priority Tags (VPT) — VPTs are mapped to an output queues based on the VPT. While queue mapping is user-
dened, the VPT default mapping to the output queue is as follows. In the VPT default mapping, Queue 1 has the lowest
priority.