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
The QoS model proceeds as follows:
Step 1
The incoming packet is classified (based on different packet fields, receive port and/or VLAN) to belong
to a traffic class.
Step 2 Depending on the traffic class, the packet is rate-limited/policed and its priority is optionally marked
(typically at the edge of the network) so that lower priority packets are dropped or marked with lower
priority in the packet fields (DSCP and CoS).
Step 3
After the packet has been marked, it is looked up for forwarding. This action obtains the transmit port
and VLAN to transmit the packet.
Step 4 The packet is classified in the output direction based on the transmit port and/or VLAN. The
classification takes into account any marking of the packet by input QoS.
Step 5
Depending on the output classification, the packet is policed, its priority is optionally (re-)marked , and
the transmit queue for the packet is determined depending on the traffic class.
Step 6 The transmit queue state is dynamically monitored via the AQM (Active Queue Management) algorithm
and drop threshold configuration to determine whether the packet should be dropped or enqueued for
transmission.
Step 7
If eligible for transmission, the packet is enqueued to a transmit queue. The transmit queue is selected
based on output QoS classification criteria. The selected queue provides the desired behavior in terms
of latency and bandwidth.
Classification
Classification is the process of distinguishing one kind of traffic from another by examining the fields
in the packet. Classification is enabled when a QoS policy-map is attached to an interface.
You specify which fields in the frame or packet that you want to use to classify incoming traffic.
For non-IP traffic, you have the following classification options:
•
CoS value in the VLAN tag of the incoming frame is used to classify the packet.
• If the frame does not contain a CoS value, the port's default CoS value ("0") is used for the
classification.
Perform the classification based on a configured MAC ACL, which examines the fields in the Layer
2 header.
For IP traffic, you have the following classification options:
•
IP DSCP or IP Precedence in the incoming packet is used for classification. DSCP values range from
0 to 63.
• Perform the classification based on a configured IP standard or extended ACL, which examines
various fields in the IP header.
Classification Based on QoS ACLs
A packet can be classified for QoS using multiple match criteria, and the classification can specify
whether the packet should match all of the specified match criteria or at least one of the match criteria.
To define a QoS classifier, you can provide the match criteria using the
match statements in a class map.
In the 'match' statements, you can specify the fields in the packet to match on, or you can use IP standard