Configuring Quality of Service
Cisco Small Business SFE/SGE Managed Switches Administration Guide 273
11
Configuring Quality of Service
Network traffic is usually unpredictable, and the only basic assurance that can be
offered is best effort traffic delivery. To overcome this challenge, Quality of
Service (QoS) is applied throughout the network. This ensures that network traffic
is prioritized according to specified criteria, and that specific traffic receives
preferential treatment. QoS in the network optimizes network performance and
entails two basic facilities:
• Classifying incoming traffic 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 traffic to a particular hardware queue
- The assignment of internal resources
- Traffic shaping
The terms Class of Service (CoS) and QoS are used in the following context:
• CoS provides varying Layer 2 traffic services. CoS refers to classification of
traffic to traffic-classes, which are handled as an aggregate whole, with no per-
flow settings. CoS is usually related to the 802.1p service that classifies flows
according to their Layer 2 priority, as set in the VLAN header.
• QoS refers to Layer 2 traffic and above. QoS handles per-flow settings, even
within a single traffic class.
The QoS facility involves the following elements:
• Access Control Lists (ACLs) — Used to decide which traffic is allowed to
enter the system, and which is to be dropped. Only traffic that meets this
criteria are subject to CoS or QoS settings. ACLs are used in QoS and network
security.
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