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Exploring QoS in Catalyst

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Discover the workings of the the various platform QoS features available across the Cisco Catalyst product family. You will gain full understanding of each Catalyst platform's supported QoS features and be able to configure various Catalyst families of switches.
This chapter is from the book

Previous chapters described the necessity for QoS in campus networks and the fundamentals behind QoS operation. This chapter explains the various platform QoS features available across the Cisco Catalyst product family. A group of concise tables in the beginning of this chapter provides a quick reference for QoS features available for each Catalyst platform. In addition, this chapter, along with subsequent chapters, begins the product tour of the access layer Catalyst switches with the fewest QoS features and continues with the high-end core Catalyst switches with industry-leading QoS features. Although the access layer switches support only a few QoS features, these switches provide an excellent foundation for exploring QoS fundamentals in the campus network.

Specifically, this chapter covers the following topics:

  • Brief Per–Catalyst Platform QoS Features Table

  • QoS Features Overview

  • QoS Features on the Catalyst 2900XL and 3500XL Switches

  • QoS Features on the Catalyst 4000 CatOS Switches

The Cisco Catalyst 2900XL, 3500XL, and 4000 Family of switches share Layer 2 QoS features needed on access layer switches. These features include the following:

  • Classification

  • Marking

  • Congestion Management

This chapter covers these topics on the respective platforms with command references, examples, and case studies. Upon completion of this chapter, you will understand each Catalyst platform's supported QoS features and be able to configure the Catalyst 2900XL, 500XL, and 4000 CatOS Family of switches for packet classification, marking, and congestion management.

From a platform perspective, the Catalyst 4000 CatOS Family of switches must be distinguished from the Catalyst 4000 Cisco IOS Family of switches due to individual differences in QoS features and configuration. The supervisor engine model determines whether a Catalyst 4000 switch operates on CatOS or Cisco IOS. In addition, the Catalyst 4000 Layer 3 services module also has exclusive Layer 3 QoS features (discussed in Chapter 7, "Advanced QoS Features Available on the Catalyst 4000 IOS Family of Switches and the Catalyst G-L3 Family of Switches"). This chapter is only applicable to the Catalyst 4000 CatOS switches. Table 3-8 shows which Catalyst 4000 switches are applicable to this chapter.

Catalyst Feature Overview

Cisco Catalyst switches support a wide range of QoS features. Generally, the high-end platforms support more QoS features especially platforms that support Layer 3 IP routing. Tables 3-1 through 3-5 provide a quick reference for QoS features for each platform. All platforms may have limitations and caveats per feature, and each QoS feature is discussed in the appropriate chapter of this book in additional detail.

Furthermore, QoS features are also dependent on whether the platform supports IP routing. The Catalyst 3550, Catalyst 4000 Cisco IOS Software family, Catalyst 5500 with Route Switch Module (RSM) or Router Switch Feature Card (RSFC), and the Catalyst 6000/6500 with Multilayer Switch Module (MSM) or Multilayer Switch Feature Card (MSFC) I/II support IP routing. Other platforms may support Layer 3 QoS features, such as classification based on differentiated services codepoint (DSCP) and marking of IP precedence; however, these platforms do not actually support routing of IP frames. As a result, network designs do not require platforms that support IP routing to classify, mark, police, or schedule traffic based on DSCP or IP precedence values. Therefore, network designers may choose lower-cost switches that do not support IP routing to enable Layer 3 QoS features.

The next sections provide quick reference tables for supported QoS features per platform. The tables only provide a glimpse into QoS feature support of each platform and do not indicate the benefits or restrictions of each feature. Refer to the appropriate chapters later in this book for thorough discussions of QoS feature support on each platform.

Specifically, the next sections highlight the following QoS features supported on each platform:

  • Input Scheduling

  • Policing

  • Classification and Marking

  • Output Scheduling

Table 3-1 indicates at a simplistic level, QoS feature support on a per-platform basis for most of the currently shipping Catalyst switches. The table only indicates at the fundamental level where a feature is supported and does not indicate the restrictions or caveats.

Table 3-1 QoS Feature Overview on Current Catalyst Switches

Product Family

Classification

Marking

Policing

Congestion Management

Congestion Avoidance

2950

Yes

Yes

Yes

Yes

No

3550

Yes

Yes

Yes

Yes

Yes

4000 IOS Family

Yes

Yes

Yes

Yes

No

6500 Family

Yes

Yes

Yes

Yes

Yes


Input Scheduling

Input scheduling is currently available only on the Catalyst 6000/6500. Input scheduling priorities and schedules packets out of ingress packet queues based on several QoS values including CoS and DSCP. However, most of Catalyst switches can deliver packets to the switching fabric at line rate or a specified rate. This specific rate defines the maximum throughput of the switch. If the input rate is not exceeded, input scheduling is not crucial in implementing QoS architecture. Furthermore, ingress policing is an option on many Catalyst switches that aids in preventing oversubscription of the switch fabric by limiting ingress traffic. Table 3-2 summarizes Catalyst platform support for input scheduling. The Comments column also denotes any switch capable of ingress policing.

Table 3-2 Catalyst Platform QoS Input Scheduling Support

Catalyst Switch

Input Scheduling

Ingress Policing

Comments

Catalyst 2900XL

No

No

Switching fabric is capable of 1.6 Gbps ingress.

Catalyst 2948G-L3/ 4912G-L3/4232-L3

No

Yes

 

Catalyst 2950

No

Yes

 

Catalyst 3500XL

No

No

Switching fabric is capable of 5.0 Gbps ingress.

Catalyst 3550

No

Yes

 

Catalyst 4000 CatOS Family

No

No

Nonblocking line cards can deliver ingress traffic at line rate to switching fabric

Catalyst 4000 Cisco IOS Family (Supervisor III and IV)

No

Yes

Non-blocking linecards can deliver ingress traffic at line rate to switching fabric.

Catalyst 5500

No

No

 

Catalyst 5500 w/NFFC1 II

No

No

 

Catalyst 6000/6500

Yes

Yes

Based on Layer 2 CoS2; option for ingress Priority Queue.


Classification and Marking

Classification and marking support and features vary per switch. Table 3-3 indicates which platforms support specific classification and marking features. All switches that support QoS also support classification based on CoS values. Current generation switches that support IP routing also support classification and marking using IP precedence or DSCP values in addition to classification and marking of CoS values.

Table 3-3 Catalyst Platform QoS Classification and Marking Support

Catalyst Switch

Classification Marking of Untagged Frames

Marking CoS on Tagged Frames

Marking DSCP on Tagged Frames

Classification Based on DSCP of Ingress Frames

Catalyst 2900XL

Yes

No

No

No

Catalyst 2948G-L3/4912G-L3/4232-L3

No

No

No

No, IP precedence only

Catalyst 2950

No, IP precedence only

Yes

Yes

Yes

Catalyst 3500XL

Yes

Yes, on specific models

No

No

Catalyst 3550

Yes

Yes

Yes

Yes

Catalyst 4000 CatOS Family

Yes

Yes

No

No

Catalyst 4000 Cisco IOS Family (Supervisor III and IV)

Yes

Yes

Yes

Yes

Catalyst 5500

Yes, requires NFFC II

Yes, requires NFFC II

Yes, requires NFFC II

Yes, requires NFFC II

Catalyst 6500

Yes

Yes

Yes

Yes


Policing

Table 3-4 indicates which Catalyst platforms support policing. Feature support and platform implementation of policing varies between each Catalyst switch. Three types of policing exist for Catalyst platforms:

  • Individual policing

  • Aggregate policing

  • Microflow policing

Individual policing applies the bandwidth limit of a policer per interface. For example, an individual policer configured to constrain ingress traffic to 32 kbps limits each applicable interface to 32 kbps on ingress. An aggregate policer configured for the same bandwidth constraint limits the bandwidth collectively among all interfaces. Microflow policing is available on the Catalyst 6500, and it applies bandwidth limits to each access-control entry (ACE) of a defined policer. Chapter 8, "QoS Support on the Catalyst 6500," discusses ACEs and microflow policing in more detail.

Each platform has unique support, restrictions, and requirements surrounding policing. Refer to each product chapter for specifics.

Table 3-4 Catalyst Platform QoS Policing Support

Cisco Catalyst Platform

Ingress Policing

Egress Policing

Individual Policing

Aggregate Policing

Microflow Policing

Catalyst 2900XL

No

No

No

No

No

Catalyst 2948G-L3/4912G-L3/4232-L3

Yes, per-port rate-limiting

Yes, per port rate-limiting and traffic shaping

No

No

No

Catalyst 2950

Yes

No

Yes

No

No

Catalyst 3500XL

No

No

No

No

No

Catalyst 3550

Yes

Yes

Yes

Yes

No

Catalyst 4000 CatOS Family

No

No

No

No

No

Catalyst 4000 Cisco IOS Family (Supervisor III and IV)

Yes

Yes

Yes

Yes

No

Catalyst 5500 w/NFFC II

No

No

No

No

No

Catalyst 6500

Yes

No

No

Yes

Yes


Congestion Management

Congestion management is supported on all Catalyst switches that support QoS features. Congestion avoidance and management is achieved via the use of output scheduling using the tail-drop and Weighted Random Early Detection (WRED) queuing mechanisms. Chapter 2, "End-to-End QoS: Quality of Service at Layer 3 and Layer 2," explains the difference between congestion management and congestion avoidance, and later chapters explain the tail-drop and WRED queuing mechanisms in the congestion avoidance section of each chapter where applicable. Moreover, only the Catalyst 3550, Catalyst 4000 IOS Family of switches, and the Catalyst 6500 support congestion avoidance.

The nomenclature for output scheduling queues is a follows:

XpYqZt
  • X indicates the number of strict-priority queues.

  • Y indicates the number of queues other than strict-priority queues.

  • Z indicates the configurable thresholds per queue.

For example, 1p3q2t indicates that a switch has an egress output queue with one strict-priority queue and three normal-priority queues each with two configurable thresholds per queue.

Table 3-5 indicates the available output queues per platform.

Table 3-5 Catalyst Platform Congestion Management Support

Cisco Catalyst Platform

Output Scheduling

Scheduling Queues

Catalyst 2900XL

Yes

Global 2q1t

Catalyst 2948G-L3/4912G-L3/4232-L3

Yes

4q

Catalyst 2950

Yes

4q

Catalyst 3500XL

Yes

Global 2q1t

Catalyst 3550

Yes

1p3q2t, 4q4t

Catalyst 4000 CatOS Family

Yes

2q1t

Catalyst 4000 Cisco IOS Family (Supervisor III and IV)

Yes

1p3q1t, 4q1t

Catalyst 5500 w/NFFC II

Yes

1q4t

Catalyst 6500

Yes

Ingress: 1q4t, 1p1q4t, 1p1q, 1p2q1t

Egress: 2q2t, 1p2q2t, 1p3q1t, 1p2q1t, 1p1q8t, and 1p1q0t


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