Cisco Catalyst 2820, Catalyst 4500 Series, Catalyst 4000 Series, Catalyst 2948G, Catalyst 2980G Frequently Asked Questions Manual

Quality of Service on Catalyst 4500/4000 Switches
Running CatOS FAQ

Document ID: 17953

Questions

Introduction
Which QoS features do Catalyst 4500/4000 switches that run CatOS support?
What is the minimum software that is necessary for QoS on Catalyst 4500/4000 switches
that run CatOS?
Do the Catalyst 4500/4000 switches that run CatOS support rate−limiting or policing on
ports or VLANs?
Can the Catalyst 4500/4000 switches that run CatOS mark or rewrite IP precedence
Type of Service (ToS) bits in an IP packet?
What kind of input scheduling do the Catalyst 4500/4000 switches that run CatOS
provide?
Can the Catalyst 4500/4000 switches that run CatOS change the incoming Class of
Service (CoS) value in IEEE 802.1Q (dot1q) tags?
My server cannot tag Class of Service (CoS) values. Can the Catalyst 4500/4000 switches
that run CatOS tag the traffic from the server for a specific CoS value?
Do the Catalyst 4500/4000 switches that run CatOS honor the incoming Class of Service
(CoS) value in IEEE 802.1Q (dot1q) tags from my IP phones?
Can the Catalyst 4500/4000 switches that run CatOS extend the trust or override the
incoming Class of Service (CoS) value of devices connected to the IP phones?
What kind of output scheduling do the Catalyst 4500/4000 switches that run CatOS
provide?
I enabled QoS on a Catalyst 4500/4000 switch that runs CatOS and now I see
performance issues. What is wrong?
How do the QoS features on the Catalyst 4500/4000 switches that run CatOS compare to
Catalyst 4500/4000 switches that run Cisco IOS Software? How do the features compare
with the features of the Layer 3 (L3) switching module?
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Related Information

Introduction

This document addresses frequently asked questions (FAQ) on the Quality of Service (QoS) feature of the
Cisco Catalyst 4500/4000 (Supervisor Engine I and Supervisor Engine II) series, Catalyst 2948G, Catalyst
2980G, and Catalyst 4912G switches that run Catalyst OS (CatOS). This document refers to these switches as
"Catalyst 4000 switches that run CatOS". For QoS features on Catalyst 4500/4000 switches that run Cisco
IOS® Software, refer to the document Configuring QoS.

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Q. Which QoS features do Catalyst 4500/4000 switches that run CatOS
support?

A. Catalyst 4500/4000 switches that run CatOS support input classification and output

scheduling on Layer 2 (L2) ports. Refer to the document Installation and Configuration Note
for the Catalyst 4000 Layer 3 Services Module for additional features that are available on
Layer 3 (L3) Gigabit Ethernet interfaces.

Q. What is the minimum software that is necessary for QoS on Catalyst
4500/4000 switches that run CatOS?

A. You need Supervisor Engine software version 5.4(2) and later on the Catalyst 4500/4000
(Supervisor Engine I and Supervisor Engine II), Catalyst 2948G, Catalyst 2980G, and
Catalyst 4912G to support the QoS features.

Q. Do the Catalyst 4500/4000 switches that run CatOS support
rate−limiting or policing on ports or VLANs?

A. Catalyst 4500/4000 switches that run CatOS do not provide policing or rate−limiting on
Layer 2 (L2) ports. Rate−limiting is supported on Layer 3 (L3) Gigabit Ethernet interfaces.
Refer to the Installation and Configuration Note for the Catalyst 4000 Layer 3 Services
Module for additional information. Policing is supported on the Catalyst 4500/4000 switches
that run Cisco IOS Software. For more information, refer to the document QOS Policing and
Marking with Catalyst 4000/4500 IOS−Based Supervisor Engines.

Q. Can the Catalyst 4500/4000 switches that run CatOS mark or rewrite IP
precedence Type of Service (ToS) bits in an IP packet?

A. Catalyst 4500/4000 switches that run CatOS do not provide Layer 3 (L3) marking or
rewriting on Layer 2 (L2) ports. The L3 ToS bits of an incoming packet are passed through
the L2 switch untouched. The incoming IP precedence on L3 module Gigabit Ethernet
interfaces is honored. Refer to the Installation and Configuration Note for the Catalyst 4000
Layer 3 Services Module for additional information. Marking and rewriting of IP
precedence/differentiated services code point (DSCP) is supported in a Catalyst 4500/4000
that runs Cisco IOS Software. For more information, refer to the document QOS Policing and
Marking with Catalyst 4000/4500 IOS−Based Supervisor Engines.

Q. What kind of input scheduling do the Catalyst 4500/4000 switches that
run CatOS provide?

A. Line cards of Catalyst 4500/4000 switches that run CatOS have FIFO input scheduling on
Layer 2 (L2) ports. Refer to the Installation and Configuration Note for the Catalyst 4000
Layer 3 Services Module for additional information on Layer 3 (L3) module features.

Q. Can the Catalyst 4500/4000 switches that run CatOS change the
incoming Class of Service (CoS) value in IEEE 802.1Q (dot1q) tags?

A. No. The Catalyst 4500/4000 switches that run CatOS support frame classification and
marking only on unclassified frames entering the switch and cannot change CoS values on
already tagged packets. Catalyst 4500/4000 switches that run Cisco IOS Software can classify
or reclassify tagged/untagged packets. For more information, refer to the document
Understanding and Configuring QoS.

Q. My server cannot tag Class of Service (CoS) values. Can the Catalyst
4500/4000 switches that run CatOS tag the traffic from the server for a
specific CoS value?

A. Yes, but remember that the default CoS for untagged packets is switch−wide and not on a
per−port basis. So, all untagged packets are marked the same CoS value. Catalyst 4500/4000
switches that run Cisco IOS Software do support tagging on a per−port basis. For more
information, refer to Understanding and Configuring QoS.

Q. Do the Catalyst 4500/4000 switches that run CatOS honor the
incoming Class of Service (CoS) value in IEEE 802.1Q (dot1q) tags from
my IP phones?

A. Yes, Catalyst 4500/4000 switches that run CatOS do honor the incoming CoS value in the
dot1q tag. Since dot1q does not tag the native VLAN, you need to use the switch−wide CoS
configuration to tag such packets. These tags are retained through the switch and used in
output scheduling. If the outgoing port is a trunk, the original CoS value or the new value (for
packets that arrive untagged on native VLAN) is tagged in the packet.

Q. Can the Catalyst 4500/4000 switches that run CatOS extend the trust
or override the incoming Class of Service (CoS) value of devices
connected to the IP phones?

A. No, Catalyst 4500/4000 switches that run CatOS cannot extend the trust or override the
incoming CoS value of traffic from devices connected to the IP phones. Catalyst 4500/4000
switches that run Cisco IOS Software can support extended trust. Refer to the document
Configuring Voice Interfaces.

Q. What kind of output scheduling do the Catalyst 4500/4000 switches
that run CatOS provide?

A. Line cards of Catalyst 4500/4000 switches that run CatOS support two output queues per
port with one threshold at 100 percent. This 2 Queues, 1 Threshold (2Q1T) method is not
configurable. It does offer user configuration for Class of Service (CoS) value maps for the
two queues. For example, you can configure packets with a CoS value of 0› to use the first
queue and 4Ÿ to use the second queue. Catalyst 4500/4000 only supports CoS mappings in
pairs, 0™, 2›, 4, 6Ÿ. You cannot set one CoS value unless you specify the
corresponding partner CoS. For example, you cannot specify 0œ for the first queue because
you must pair 5 with the partner, 4. The two queues are serviced in a round−robin fashion.
Refer to the Installation and Configuration Note for the Catalyst 4000 Layer 3 Services
Module for additional information on Layer 3 module features. For Catalyst 4500/4000
switches that run Cisco IOS Software features, refer to the document Understanding and
Configuring QoS.

Q. I enabled QoS on a Catalyst 4500/4000 switch that runs CatOS and
now I see performance issues. What is wrong?

A. When QoS is disabled, unicast traffic is assigned to queue 1 and broadcast, multicast, and
unknown traffic is assigned to queue 2. If you enable QoS but do not modify the Class of
Service (CoS)−to−transmit queue mappings, switch performance can be affected because all

traffic is assigned to queue 1. If you enable QoS, modify the CoS−to−transmit queue
mappings.

Q. How do the QoS features on the Catalyst 4500/4000 switches that run
CatOS compare to Catalyst 4500/4000 switches that run Cisco IOS
Software? How do the features compare with the features of the Layer 3
(L3) switching module?

A. The Catalyst 4500/4000 is available in three configurations that vary with the supported
QoS features. This table summarizes these variations:

Supervisor
Engine I/II

(Which

Includes L2

1

Ports on an

L3 Module

That Runs

CatOS)

WS−X4232−L3

Module (L3

Gigabit Ports

Only)

Cisco IOS

Software

(Supervisor

Engine II+,

III, IV, and

V)

MQC

2

support

No No Yes
Switch−wide
QoS

Yes N/A Yes
Per−port QoS

No Yes Yes
Transmit
queues per
port

2Q1T3Map

CoS4 values

to queues

with the set

qos map

command

4Q

5

4Q

Receive
queues per
port

N/A N/A N/A

6

Scheduling

Round−robin

WRR7 with the

qos mapping
precedence value
wrr−weight

weight command

Round−robin,
WRR, or strict

priority
Strict priority
queue

No No

Yes, with the

tx−queue 3

priority high

command
Congestion

avoidance

No No

Yes, DBL8 is

available on

Supervisor

Engine IV
Policers

(ingress)

N/A Yes, with the

rate−limit input

Yes, up to 1K

policers

9

command

Policers
(egress)

N/A

Yes, with the

rate−limit
output command

Yes, up to 1K

policers

9

Input/output
policer
definition
with L3 and
L410 headers

No

No, applies to all
IP and non−IP
traffic per port

Yes

Output
shaping

No

Yes, as of Cisco
IOS Software
Release

12.0(10)W5(18e)
with the

traffic−shape
rate command

Yes,

per−egress

queue with the

shape

command

Classification
with an IP
DSCP11 basis

No

Yes, IP
precedence bits
only

Yes, on the

basis of

"trusted"

values of

arriving

packets or via

the

configuration

of per−port,

ACL−based12,

or class−based

marking
Classification

based on
IEEE 802.1p
(CoS)

Yes N/A

Yes, based on

"trusted"

values of

arriving

packets or via

configured

marking rules
Classification
based on
ACLs or
traffic classes

No

No, applies to all
IP and non−IP
traffic, except
high−priority
packets destined
to the CPU

Yes

Marking
based on
ISL13, 802.1p,
and IP ToS

14

Yes,
switchwide
settings only
with set qos

defaultcos

command and
only on
unclassified
or untagged

No, honors
existing IP
precedence values
for input
classification and
output
scheduling.

Yes

frames

1

L2 = Layer 2

2

MQC = Modular QoS command−line interface (CLI)

3

2Q1T = 2 Queues, 1 Threshold

4

CoS = Class of Service

5

4Q = 4 Queues

6

The Supervisor Engine provides nonblocking switch architecture to obviate the need for

input queuing.

7

WRR = weighted round−robin

8

DBL = Dynamic Buffer Limiting

9

Be aware of Cisco bug ID CSCdz48041 (

registered customers only

) , which might cause

exhaustion of tags for policers when configuring policers on many interfaces.

10

L4 = Layer 4

11

DSCP = differentiated services code point

12

ACL = access control list

13

ISL = Inter−Switch Link Protocol

14

ToS = Type of Service

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Related Information

QOS Policing and Marking with Catalyst 4000/4500 IOS−Based Supervisor Engines•
Catalyst G−L3 Series Switches and WS−X4232−L3 Layer 3 Modules QoS FAQ•
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Updated: Nov 16, 2007 Document ID: 17953