68 Chapter 3: Overview of QOS Support on Catalyst Platforms
From a platform perspective, the Catalyst 4000 CatOS Family of switches must be distin-
guished 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 classifi-
cation 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
Catalyst Feature Overview 69
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 funda-
mental level where a feature is supported and does not indicate the restrictions or caveats.
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 3-1 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
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
continues
70 Chapter 3: Overview of QOS Support on Catalyst Platforms
*NFFC = NetFlow Feature Card
**CoS = class of service
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.
Catalyst Switch Input Scheduling Ingress Policing Comments
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/NFFC* II
No No
Catalyst 6000/6500 Yes Yes Based on Layer 2
CoS**; option for
ingress Priority
Queue.
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
Table 3-2 Catalyst Platform QoS Input Scheduling Support (Continued)
Catalyst Feature Overview 71
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.
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 5500 Yes, requires NFFC II Yes, requires
NFFC II
Yes, requires
NFFC II
Yes, requires NFFC II
Catalyst 6500 Yes Yes Yes Yes
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
Table 3-3 Catalyst Platform QoS Classification and Marking Support (Continued)
continues
72 Chapter 3: Overview of QOS Support on Catalyst Platforms
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.
Cisco Catalyst
Platform
Ingress
Policing Egress Policing
Individual
Policing
Aggregate
Policing
Microflow
Policing
Catalyst 4000
Cisco IOS Family
(Supervisor III and IV)
Yes Ye s Yes Ye s No
Catalyst 5500
w/NFFC II
No No No No No
Catalyst 6500 Yes No No Yes Yes
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
Table 3-4 Catalyst Platform QoS Policing Support (Continued)
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