Cisco Catalyst QoS: Quality of Service in Campus Networks

74 Chapter 3: Overview of QOS Support on Catalyst Platforms

Figure 3-1 General Catalyst QoS Packet-Flow Architecture

QoS Support on the Catalyst 2900XL and 3500XL

Specific models of the Catalyst 2900XL and 3500XL support QoS classification and

congestion management. Ingress packet CoS values and configured port priorities exclu-

sively determine classification of ingress frames for placement into either a low-priority or

high-priority global transmit queue. The two global transmit queues with priority sched-

uling create the congestion management mechanism. The following sections discuss these

QoS features with detailed overviews, configuration guidelines, and examples.

General Catalyst Switch CoS Architecture

Ingress Packet

Classification

Input Scheduling

Layer 2/3 Lookup

Policing

Rewrite

Output

Scheduling

Egress Packet

QoS Support on the Catalyst 2900XL and 3500XL 75

Catalyst 2900XL Product Family Delineation

QoS support on the Catalyst 2900XL and 3500XL platforms is software and model

dependent. For the 2900XL, the original-edition models do not support QoS features,

including the uplink modules for the Catalyst 2916M. All standard- and enterprise-edition

models do support QoS features. Table 3-6 indicates which 2900XL models support QoS

features. All models of the 3500XL support QoS features. In addition, the Catalyst

3524XL-PWR-XL and the 3548XL support CoS reclassification. This QoS feature is not

available on other 3500XL platforms.

Catalyst 2900XL and 3500XL QoS Architectural Overview

The Catalyst 2900XL and 3500XL switches are limited to QoS features that suit access

layer switches. These features include classification, marking, and congestion management

via the use of output scheduling. Because of these features, the 2900XL and 3500XL fit

well into an end-to-end QoS design with core switches. Figure 3-2 shows a sample network

deploying access layer QoS features with comprehensive QoS features in the core.

Table 3-6 QoS Support by Model of 2900XL

Catalyst 2900XL/

3500XL Model Description

QoS

Support

WS-C2908-XL 8-port 10/100BASE-TX switch No

WS-C2912-XL-A/EN 12-port 10/100BASE-TX switch Yes

WS-C2912MF-XL 12-port 100BASE-FX switch Yes

WS-C2916M-XL 16-port 10/100BASE-TX switch + 2 uplink slots No

WS-C2924-XL 24-port 10/100BASE-TX switch No

WS-C2924C-XL 22-port 10/100BASE-TX + 2-port 100BASE-FX switch No

WS-C2924-XL-A/EN 24-port 10/100BASE-TX switch Yes

WS-C2924C-XL-A/EN 22-port 10/100BASE-TX switch + 2-port 100BASE-FX

switch

Yes

WS-C2924M-XL-A/EN 24-port 10/100BASE-TX switch + 2 uplink slots Yes

WS-C2924M-XL-EN-DC 24-port 10/100BASE-TX switch + 2 uplink slots (DC

power)

Yes

76 Chapter 3: Overview of QOS Support on Catalyst Platforms

Figure 3-2 Network Topology Using Catalyst 3524XLs

Figure 3-3 shows the basic QoS architecture model for the Catalyst 2900XL and 3500XL

discussed in the following sections.

Software Requirements

For QoS feature support, the 2900XL and 3500XL require Cisco IOS Software Release

12.0(5)XP or higher. The 3524-PWR-XL and the 3548XL require 12.0(5)XU or higher for

the reclassification of class of CoS values in frames.

Input Scheduling

The Catalyst 2900XL and 3500XL do not perform input scheduling as ingress packets are

immediately copied to a global, shared memory buffer. As long as the packet-forwarding

rate of the switch is not exceeded, input congestion is not critical to implementing QoS. The

packet-forwarding rates of the Catalyst 2900XL and the Catalyst 3500XL are 1.6 Gbps and

5.0 Gbps, respectively.

Catalyst 6500

with MSFC II

Catalyst 6500

with MSFC II

Cisco IP

Phones

Workstations

Cisco IP

Phones

Workstations

Catalyst 3524-

PWR-XL

Catalyst 3524-

PWR-XL

QoS Support on the Catalyst 2900XL and 3500XL 77

Figure 3-3 Basic QoS Architecture for the Catalyst 2900XLs and 3500XLs Switches

Classification/Reclassification

The Catalyst 2900XL and 3500XL switches both support classification of untagged frames.

Two models of the Catalyst 3500XL switch, the Catalyst 3524-PWR-XL and the Catalyst

3548XL switches, support marking of ingress tagged frames. Classification and marking is

configurable only on a per-port basis, and each port may be configured with a unique CoS

value to be classified.

To configure a Catalyst 2900XL and 3500XL for classification or marking of frames, use

the following interface command:

switchport priority {default

default-priority-id

|

extend {cos

value

| none |

trust} |

override}

• The default-priority-id parameter is the CoS value to be assigned to untagged ingress

frames.

• The extend option is to configure the 802.1p trust configuration of the connected

appliance on the P1 port of the IP Phone. For example, a Cisco IP Phone can be

configured to trust or reclassify frames received on its P1 port.

Classification

CoS = 4-7

Place Packet in

Global Low

Priority Queue

Place Packet in

Global High

Priority Queue

Schedule

L2 Lookup

Transmit Packet

Ye sNo