cisco 2950 qos cos 说明

Introduction

This document addresses the Frequently Asked Questions (FAQs) on the Quality of Service (QoS)

features of the Cisco Catalyst 2950 series switches.

Q. What is the software version requirement for the QoS features on Catalyst 2950 series

switches?

A.

The 2950 supports egress queuing and scheduling based on IEEE 802.1p class of service (CoS)

values with Cisco IOS?

Software Release 12.0(5)WC(1) and later. Each port on the switch gets a

default priority value. Frames that are received untagged are assigned this value. For those frames

that are received tagged, the 2950 uses the value in the tag. The 2950 queues frames at the egress

port into one of four priority queues. The switch makes the queue assignments on the basis of the

priority or CoS value that is assigned to the frame at ingress. You can configure egress scheduling as

either strict priority or weighted round-robin (WRR) scheduling. The Catalyst 2950 series switches

run two feature sets of Cisco IOS Software, a standard image (SI) and an enhanced image (EI). Several

feature differences exist between the SI version and the EI version. The SI only supports the output

scheduling QoS features, whereas the EI adds support for classification, marking, and policing. Refer

to the

Release Notes for the Catalyst 2955, Catalyst 2950, and Catalyst 2940 Switches, Cisco IOS

Release 12.1(22)EA4

for hardware and software compatibility.

Q. What QoS features does the 2950 standard image (SI) support?

A.

The 2950 with the SI supports queuing and scheduling at egress. The 2950 with SI supports

ingress classification with use of port trust states in Cisco IOS Software Release 12.1(11)EA1 and

later. You can configure the ingress port to trust either class of service (CoS) or differentiated

services code point (DSCP), where the default port trust state is untrusted. You can configure egress

scheduling as either strict priority scheduling or weighted round-robin (WRR) scheduling.

In Cisco IOS Software Releases 12.0, SI only supports configuration on CoS priorities and WRR. In

Cisco IOS Software Releases 12.1 and later, SI added these features:

?

Classification

o

Classification on the basis of Cisco Discovery Protocol (CDP) from a Cisco IP

phone

Output queuing and scheduling

o

Strict priority scheduling

o

WRR scheduling

?

Q. What QoS features does the 2950 enhanced image (EI) support?

A.

The 2950 with the EI supports Layer 2 (L2)

–

Layer 4 (L4) classification at ingress with use of:

?

?

?

Port trust states

QoS access control lists (ACLs)

Class maps and policy maps

The 2950 with EI also supports policing and marking at ingress as well as queuing and scheduling at

egress. You can configure egress scheduling as either strict priority scheduling or weighted round-

robin (WRR) scheduling. Refer to the

Release Notes for the Catalyst 2955, Catalyst 2950, and Catalyst

2940 Switches, Cisco IOS Release 12.1(22)EA4

for hardware and software compatibility.

In Cisco IOS Software Releases 12.0, EI only supports configuration on class of service (CoS)

priorities and WRR. In Cisco IOS Software Releases 12.1, EI added these features:

?

Classification

o

Classification on the basis of:

?

Port trust state

?

Access control lists (ACLs)

?

Policy maps

?

Cisco Discovery Protocol (CDP) from a Cisco IP phone

o

Trust of CoS/differentiated services code point (DSCP)

o

Ports CoS configuration

Marking

Policing

o

Ingress policing

Configuration of the mapping table

o

CoS-to- DSCP

o

DSCP-to- CoS

Output queuing and scheduling

o

Strict priority scheduling

o

WRR scheduling

?

?

?

?

Q. Do the Catalyst 2950 series switches support rate- limiting or policing on ports or VLANs?

A.

Catalyst 2950 series switches that run the enhanced image (EI) support ingress policing on

physical interfaces only. Policers do not have support on VLAN interfaces. Catalyst 2950 series

switches that run the standard image (SI) do not support policing. The minimum release with

support is Cisco IOS Software Release 12.1.

Q. Can the Catalyst 2950 series switches mark or rewrite IP precedence (type of service [ToS])

bits in an IP packet?

A.

Yes, the Catalyst 2950 series switches that run the enhanced image (EI) can mark or rewrite ToS

bits in the header of an IP version 4 (IPv4) packet. Use a policy map that contains the

set ip

dscp

statement. Or configure a policer to mark down or rewrite the differentiated services code point

(DSCP) value on frames that do not conform to the rules in the policer.

Note:

The Catalyst 2950 only supports ingress policing.

Q. Do the Catalyst 2950 series switches provide priority scheduling in the input/ingress port?

A.

Catalyst 2950 series switches do not provide any ingress scheduling, but they do support egress

queuing and scheduling. Use the

wrr-queue bandwidth

command. The default is to use FIFO.

Q. Do the Catalyst 2950 series switches honor the incoming class of service (CoS) values in

IEEE 802.1p (dot1p) tags from IP phones?

A.

The Catalyst 2950 series switches provide QoS-based 802.1p CoS values. In Cisco IOS Software

Release 12.0(5)WC1 and later, the 2950 series switches honor the CoS value of an incoming frame by

default.

Note:

For untagged frames that are received on trusted and untrusted ports, QoS assigns the CoS

value that the

mls qos cos

interface configuration command specifies. By default, this value is zero,

which is trusted.

Q. My server or IP phone/device cannot tag class of service (CoS) values. Can the Catalyst 2950

series switches tag the traffic from the server/device for a specific CoS value?

A.

Yes, the switch can tag the traffic. But you must configure the port as a trunk port and

configure

mls qos cos

value

, as well. In this sample configuration, interface FastEthernet 0/1

connects to an IP phone that cannot tag CoS values:

interface fastethernet 0/1

switchport mode trunk

mls qos cos 7

For IEEE 802.1Q frames with tag information, the priority value from the header frame is used. For

frames that are received on the native VLAN, the default priority of the input port is used.

Q. Can I override the incoming class of service (CoS) to a specific CoS value?

A.

Yes, you can override the CoS. Issue the

mls qos cos {

default- cos

| override}

command. The

command overrides the previously configured trust state of the incoming packets and applies the

default port CoS value to all incoming packets. Cisco IOS Software Release 12.1 and later support this

command.

Q. What is pass-through mode?

A.

In pass-through mode, the switch uses the class of service (CoS) value of incoming packets without

a modification of the differentiated services code point (DSCP) value. The frame can pass through the

switch with both the incoming CoS and DSCP values intact. When you disable pass-through mode and

configure the switch port to trust CoS, the DSCP value is derived from the CoS- to-DSCP map. In this

case, the DSCP usually changes as a result. In Cisco IOS Software releases earlier than Cisco IOS

Software Release 12.1(11)EA1, this derivation of the DSCP value is on by default and you cannot

change it. In Cisco IOS Software Release 12.1(11)EA1 and later, you can configure this with the

enablement of pass-through mode on the port.

Here is a sample configuration:

interface fastethernet 0/1

switchport mode access

mls qos trust cos pass-through dscp

Q. Can I reclassify the class of service (CoS) value of data that are generated from a PC that

connects to an IP phone? The IP phone attaches to a Catalyst 2950 series switch.

A.

Yes, you can reclassify the CoS value of the data. Issue the

switchport priority extend

cos

interface configuration command. The command configures the IP phone to override the priority

of the traffic that comes from the PC.

Q. What kind of output scheduling do the Catalyst 2950 series switches provide?