QoS Service Class (Classification, Marking, and NBAR)

Planning and implementing QoS policies entails three main steps:

Step 1 Identify network traffic and its requirements.

Step 2 Divide the identified traffic into classes.

Step 3 Define QoS policies for each class.

In Step 1, you use tools such as NBAR to identify the existing traffic in the network. You might discover many different traffic types. In Step 1, you must then recognize and document the relevance and importance of each recognized traffic type to your business.

In Step 2, you group the network traffic into traffic or service classes. Each traffic or service class, composed of one or more traffic types, receives a specific QoS treatment. Each service class is created for one or more traffic types (a single group) that is called a BA. A common model used by service providers, called the customer model, defines four service classes:

■ Mission critical

■ Transactional

■ Best-effort

■ Scavenger

A traffic class can be defined based on many factors. For example, these criteria, should they be appropriate, can also be used to define traffic classes: an organization or department, a customer (or a set of them), an application (or a group of applications, such as Telnet, FTP, SAP, Oracle), a user or group of users (by location, job description, workstation MAC address), a traffic destination, and so on.

Step 3 in planning and implementing QoS policies using QoS service classes is defining policies for each service class. This step requires an understanding of the QoS needs of the traffic and applications that are within your network. When you design the policies, be careful not to make too many classes and make the matter too complex and over-provisioned. Limiting the service classes to four or five is common. Also, do not assign too many applications and traffic to the high-priority and mission-critical classes, because assigning a large percentage of traffic to those classes will ultimately have a negative effect. Some of the existing common traffic classes are as follows:

■ Voice applications (VoIP)

■ Mission-critical applications, such as Oracle and SAP

■ Transactional/Interactive applications, such as Telnet and SSH

■ Bulk applications such as FTP and TFTP

■ Best-effort applications, such as WWW and e-mail

■ Scavenger applications, such as Napster and Kazaa

You can find many sources of information and recommendations on QoS design and implementation; however, each network is unique and requires special attention. It is important to implement the QoS policies throughout the network and in a consistent way. Keep in mind the following two important points:

■ If you do not implement QoS policies in certain parts of the network, the QoS offering of your network will be incomplete, unpredictable, and inadequate.

■ Because not all network devices have consistent and complete capabilities and features, you must map QoS techniques and features well. That way, the behavior of the diverse devices within your network will be consistent and in-line with your policies.

One required task during the QoS policy implementation stage is mapping and translating between CoS, DSCP, IP precedence, and MPLS EXP markings. Table 3-4 shows the Cisco recommended mappings between Layer 2 CoS, IP precedence, DSCP, PHB and Class Selector Name, and their corresponding traffic types.

Table 3-4 Mapping Different Markings to Different Traffic Types

Table 3-4 Mapping Different Markings to Different Traffic Types