Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 44 Configuring Quality of Service
Overview of QoS
You create and name a policy map by using the policy-map global configuration command. When you
enter this command, the switch enters the policy-map configuration mode. In this mode, you specify the
actions to take on a specific traffic class by using the set,
police , bandwidth , or shape policy-map
configuration and policy-map class configuration commands. To make the policy map effective, you
attach it to an interface by using the
service-policy interface configuration command.
The policy map can also contain commands that define the policer, (the bandwidth limitations of the
traffic) and the action to take if the limits are exceeded. For more information, see the
“Policing and
Marking” section on page 44-8 .
A policy map also has these characteristics:
• A policy map can contain up to 254 class statements.
•
You can have different classes within a policy map.
Policing and Marking
Policing involves creating a policer that specifies the bandwidth limits for the traffic. Packets that exceed
the limits are
out of profile or nonconforming . Each policer specifies the action to take for packets that
are in or out of profile. These actions, carried out by the marker, include passing through the packet
without modification, dropping the packet, or marking down the packet with a new DSCP value that is
obtained from the configurable policed-DSCP map. You can configure policer within a policy map with
the
police command in policy-map class configuration mode. For information on the policed-DSCP map,
see the “Queueing and Scheduling” section on page 44-8 .
When configuring policing and policers, keep these items in mind:
• Policers account only for the Layer 2 header length when calculating policer rates. In contrast,
shapers account for header length as well as IPG in rate calculations.
•
Beginning with Cisco IOS Release 15.0(2)SG (IOS XE 3.2.0), Supervisor Engine 6-E, Supervisor
Engine 6L-E, Catalyst 4900M, Catalyst 4948E, Supervisor Engine 7-E, and Supervisor Engine 7L-E
support the
qos account layer-all encapsulation command, which accounts for Layer 1 headers of
20 bytes (12 bytes pr 8 bytes IPG) and Layer 2 headers in policing features.
• Only the average rate and committed burst parameters are configurable.
•
After you configure the policy map and policing actions, attach the policy to an ingress or egress
interface by using the
service-policy interface configuration command.
• For 2 rate 3 colors (2r3c) policers, if no explicit violation-action is specified, the exceed-action is
used as the violate-action.
Queueing and Scheduling
The Catalyst 4500 series switch supports 8 transmit queues per port. Once the decision has been made
to forward a packet out a port, the output QoS classification determines the transmit queue into which
the packet must be enqueued.
Queues are assigned when an output policy attached to a port with one or more queuing related actions
for one or more classes of traffic. Because there are only eight queues per port, there are at most eight
traffic classes (including
class-default, the reserved class) with queuing action(s). Classes of traffic that
do not have any queuing action are referred to as non-queuing classes. Non-queuing class traffic use the
queue corresponding to
class-default .