QoS配置基础

Configuration of QoS

In last month's newsletter I discussed the mechanisms QoS provides for

delivering traffic with different priorities and assurances. This month

we are going to look at the specific configurations for enabling the

QoS assurances. Configuration of QoS is somewhat confusing as there are

many types of QoS "knobs" that can be configured on a router or a

switch. Hopefully this newsletter will provide the necessary

instruction for getting through some of the complexities of QoS

configurations. This newsletter focuses on the QoS configurations

relevant to Cisco IOS not CatOS. Layer2 QoS is another topic entirely

and will be reserved for future newsletters.

To refresh our memory from last month, let's review QoS briefly. QoS is

a set of tools, or "knobs" as they are sometimes called, that are

configured on network elements (usually routers and switches) to

enforce traffic delivery policies for certain types of traffic. QoS can

be broken down into three levels or models to provide end-to-end levels

of service to traffic delivered over the network infrastructure. These

models are called best-effort service, integrated service and

differentiated service. Within these models, QoS defines different

techniques to handle traffic. These techniques are congestion

avoidance, congestion management, policing and traffic shaping. We will

focus on the configuration of the congestion management techniques in

this newsletter. Congestion avoidance, policing and traffic shaping

techniques will be discussed next month.

Congestion Management

Congestion management techniques control the prioritization of traffic

as it leaves the queues, it does not minimize congestion. Cisco

provides several different techniques for providing congestion

management. The mechanisms include First In First Out Queuing (FIFO),

Weighted Fair Queuing (WFQ), Priority Queuing and Custom Queuing. In

addition, there is Class Based Weighted Fair Queuing (CBWFQ), IP RTP

Priority (or Priority Queue WFQ) and Low Latency Queuing (LLC).

First in First Out Queuing (FIFO)

FIFO has no concept of priority or classes of traffic and does not

provide any prioritization of packets

Weighted Fair Queueing (WFQ)

WFQ provides allocation of bandwidth to network conversations. Weights

are assigned based on source and destination patterns. WFQ

automatically gives precedence for low-volume traffic such as telnet

and HTTP over high volume traffic such as FTP. WFQ is enabled by

default on interfaces that run below E1 speeds (2.048 Mbps).

Priority Queuing (PQ)

PQ allows you to define four queues for traffic prioritization. Traffic

is matched using match criteria such as IP address, port numbers and

interfaces. The four priorities are defined as high, medium, normal and

low. The queues are served in a preferential manner. The router will

not service the lower-priority queues until all packets from the

higher-priority queues have been delivered. This can be detrimental if

not configured properly as all of the bandwidth can be consumed by the

higher-priority queues, not allowing the router to transmit data out of

the other queues. Let's say you want to do the following for traffic

being forwarded out interface serial 0.

Telnet (TCP port 23) traffic is assigned to the high-priority queue.

AppleTalk and IPX are given the medium-priority queue.

All other IP traffic is assigned to the normal-priority queue.

All other traffic is assigned to the low-priority queue (as default)

Below is the configuration:

priority-list 2 protocol ip high tcp 23

priority-list 2 protocol ip high list 1

priority-list 2 protocol interface ethernet 0 medium

priority-list 2 protocol ip normal

priority-list 2 queue-limit 15 20 20 30

!

access-list 1 permit 131.108.0.0 0.0.255.255

!

interface serial 0

priority-group 2

You can change the default number of packets in each queue by using the

following command:

Priority-list list-number queue-limit high-limit medium-limit normal-

limit low-limit

This is not recommended!!!!!

Custom Queuing (CQ)

CQ provides the same type of classification of packets as PQ with 16

queues instead of four. In addition, CQ allows the number of bytes to

be specified for forwarding each time the queue is serviced. The queues

are serviced in a round-robin fashion. By servicing each queue in a

round-robin fashion, CQ ensures that no application receives more than

a predetermined amount of overall bandwidth. Let's say you want to do

the following for traffic being forwarded out interface serial 0.

Traffic from E0 is assigned to queue 1.

IP traffic is sent to queue 2.

IPX traffic goes to queue 3.

AppleTalk traffic goes to queue 4

Queue 5 is the default queue. Traffic that is not from E0 or is not

IP-, IPX- or AppleTalk-based is sent to this queue.

The configuration would be as follows:

queue-list 1 interface E0 1

queue-list 1 protocol ip 2

queue-list 1 protocol ipx 3

queue-list 1 protocol appletalk 4

queue-list 1 default 5

!

Interface serial 0

custom-queue-list 1

CQ allows for granularity on the number of packets in any queue and the

number of bytes delivered from a queue. This is done by using the

following commands:

queue-list 1 queue 10 limit 40 (limit number of packets)

queue-list 1 queue 10 byte-count 1400 (set byte count)

In addition, access lists can be used to identify what applications are

serviced by each queue. Instead of just configuring IP in queue 2 you

could add a protocol number or access list to the queue list command.

For example:

queue-list 1 protocol ip 2 TCP 23 (TCP) OR

queue-list 1 protocol ip 2 list 10 (where 10 is an access list defining

certain traffic types)

More Congestion Management Techniques

The last three types of congestion management techniques are presented

separately as they utilize enhancements and combinations of the

previous queuing techniques. The following queuing techniques are used

primarily in IP telephony environments as they have the ability to give

more granular prioritization to voice traffic.

Class Based Weighted Fair Queuing (CBWFQ)

CBWFQ extends WFQ to provide user-defined classes using match criteria.

Queues are reserved for each class, and characteristics are assigned to

each queue. The characteristics include bandwidth, weight and queue

limit. The way CBWFQ works is as follows: Match criteria filters and

classify packets first and then each packet is assigned a weight. The

weight is derived from the bandwidth you assigned to the class.

Finally, each packet is placed in the appropriate queue and serviced

according to the weights. In order to configure this on a Cisco router,

you must perform the following three processes:

* Class Map Process: Define the traffic classes (i.e., what packets

are going to be matched and by what criteria).

* Class Policy Process: Specify the policies for each clas