Configuring Quality of Service
By using the quality of service (QoS) feature, you can give preferential service
to selected packets. When QoS is not enabled on a device, the device offers
best-effort service to each packet, regardless of the packet contents or size.
When QoS is configured and enabled, you have much more control over network
traffic. For example, you can:
- Classify traffic
- Prioritize traffic
- Allocate dedicated bandwidth to specific traffic types
- Manage network congestion
QoS is always enabled on Catalyst 2950 switches. To enable QoS on Catalyst
3550 switches, choose Device > QoS > Enable/Disable and use the
QoS Enable/Disable window.
Before you use QoS, you must perform these tasks:
- Decide which marker in incoming packets to trust. Incoming
packets can contain any or none of these markers: a class
of service (CoS) value, 0 to 7; an IP precedence
value, 0 to 7; a Differentiated Services Code
Point (DSCP) value, 0 to 63. You decide which of these marker types you
want to trust
that is, which
one you want to use to classify incoming packets on the interface. You also
decide what default value (always a CoS value) to assign if a packet contains
no marker. To perform this task, you choose Device > QoS > Trust
Settings and use the QoS
Trust Settings window.
Restriction: For Catalyst 2950 switches, you can set interfaces to
trust only CoS and DSCP values.
- Map markers to DSCP and to CoS. You might decide to trust CoS or
IP Precedence on a given interface, but the device uses only DSCP values to
associate packets with queue thresholds.
Therefore, you must fill in tables that map CoS and IP precedence values to
DSCP values. Also, the device uses only CoS values to assign packets to egress
queues, so you must fill in another table that maps DSCP values to CoS values.
To perform these tasks, you choose Device > QoS > Maps and use
the QoS Maps window. This window
also has features that let you:
- Mutate DSCP values; that is, you can convert any set of DSCP
values to a different set of DSCP values.
- Define how traffic on the interface is to be marked down (demoted in
priority) if it does not adhere to size and burst rate limitations.
Restriction: QoS maps are supported on Catalyst 2950 switches only
if they are running the enhanced software image.
Even then, only two mappings are supported: CoS to DSCP and DSCP to CoS.
- Configure queues on device interfaces. Each device interface has
four egress queues, and you make a number of decisions about their characteristics.
For instance:
- How much of the available storage is allotted to each queue
- How much of the available bandwidth is allotted to each queue
- Whether one of the queues is designated as a high-priority queue
- What thresholds apply to each queue
- What DSCP values map to each threshold on each queue
- Which packets are assigned (by CoS value) to each queue
Note: You can change these assignments on Catalyst 2950 and Catalyst
3550 switches.
To perform this task, you choose Device > QoS > Queues and use
the QoS Queues window.
- Create policers. A policer is a specification that contains a maximum
permissible rate of transmission, a maximum burst size for transmissions,
and an action to take if either maximum is exceeded. You associate a policer
with a QoS class. Ultimately, you will associate
one or more classes with a policy and attach
policies to interfaces. In this way, policers set transmission and burst limits,
per class of packets, on an interface.
You can create aggregate policers and nonaggregate
policers. To create an aggregate policer, you choose Device > QoS >
Aggregate Policers and click Create in the QoS
Aggregate Policers window. You create a nonaggregate policer in the process
of adding a class to a policy.
Restriction: QoS policers are supported on Catalyst 2950 switches only
if they are running the enhanced software image. Even then, only nonaggregate
policers are supported.
- Create classes. A class identifies packets that have a certain DSCP
or IP precedence value or that match a filtering criterion in a certain ACL.
When a class is specified in a policy, it is paired with a policer. To create
classes, you choose Device > QoS > Classes and click Create
in the QoS Classes window.
Restriction: QoS classes are supported on Catalyst 2950 switches only
if they are running the enhanced software image. Even then, a class can be
defined to match an ACL but not to match a DSCP or IP precedence value.
- Create policies. A policy is a set of one or more classes and their
associated policers. When you create a policy, you can specify classes and
policers that already exist (the results of tasks 5 and 6), or you can define
classes and policers in the process of creating the policy. To create policies,
you choose Device > QoS > Policies and click Create in
the QoS Policies window.
Restriction: QoS policies are supported on Catalyst 2950 switches only
if they are running the enhanced software image.
- Attach policies to interfaces. You can use a policy to regulate input,
output, or both, on an interface, and you can use the same or different policies
with different interfaces. When you attach a policy to an interface, it is
in effect, assuming QoS is enabled on the device. To attach policies to interfaces,
you choose Device > QoS > Policies and click Attach/Detach
in the QoS Policies window.
If a Catalyst 3550 switch is in the cluster, you can use the QoS
Statistics window to specify the devices, interfaces, and packet types for
which QoS statistics are kept. Your specifications determine the statistics
that are available in the QoS Reports
window and QoS Graphs window.
Related Web Links
"Configuring
QoS," Catalyst 2950 Desktop Switch Software Configuration Guide
"Configuring
QoS," Catalyst 3550 Multilayer Switch Software Configuration Guide