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Chapter30 Co nf iguring QoS
Understanding QoS
You can also use these wizards to configure QoS only if your switch is running the EI:
•Priority data wizard—Lets you assign priority levels to data applications based on their TCP or UDP
ports. It has a standard list of applications, and you se lec t t he o ne s tha t y ou want to pri ori tize , the
priority levels, and the interfaces where the prioritization occurs. Refer to the priority data wizard
online help for procedures about using this wizard .
•Video wizard—Gives traffic that originates from specified video servers a higher priority than the
priority of data traffic. The wizard assumes that the video servers are connected to a single device
in the cluster. Refer to the video wizard online help for procedures about using this wizard.
This chapter consists of these sections:
•Understanding QoS, page 30-2
•Configuring Auto-QoS, page 30-9
•Displaying Auto-QoS Information, page 30-15
•Auto-QoS Configuration Example, page 30-16
•Configuring Standard QoS, page 30-18
•Displaying Standard QoS Information, page 30-39
•Standard QoS Configuration Examples, page 30-39
Understanding QoS
This section describes how QoS is implemented on the switch. If y ou have the SI i nsta ll ed on yo ur
switch, some concepts and features in this section might not apply. For a list of available features, see
Table30-1 on pag e 30-1.
Typically, networks operate on a best-ef fort de liv ery basis, which mean s that all t raf fic h as equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an
equal chance of being dropped.
When you configure the QoS feature, you can select specific network traffic, prioritize it according to
its relative importance, and use congestion-management and conge stio n-avoidance t ech nique s to give
preferential treatment. Implementing QoS in your network makes network performance mor e
predictable and bandwidth utilization more effective.
The QoS implementation is based on the DiffServ archit ect u re, a n em erg ing s tan da rd fr om the In tern et
Engineering Task Force (IETF). This architecture spe cif ies that each pack et i s classi f ied u pon entr y into
the network. The classification is carried in the IP packet header, using 6 bits from the deprecated IP
type-of-service (ToS) field to carry the classification (class) information.
Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or
a La yer 3 packet are described here and shown in Figure 30-1:
•Prioritization values in Layer 2 frames
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the class of
service (CoS) value in the three most-significant bits, which are called the User Priority bits. On
interfaces configured as Layer 2 802.1Q trunks, all traffic is in 802.1Q f ra mes excep t fo r t raffic in
the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 for low priority to 7 for high priority.