Chapter 8. Understanding Quality of Service on Catalyst 6500

This chapter covers the following topics:

  • QoS services

  • Class of service

  • Type of service

  • QoS operational model

  • QoS caveats

Quality of service (QoS), at a rudimentary level, is defined as providing preferential treatment for priority traffic. It does this on the expense of low-priority traffic. The motivation behind QoS is to allow network- and user-critical data to have consistent and reliable access to the network. QoS deployment is central to converged networks where IP telephony has merged with traditional data network. QoS helps with management of latency and bandwidth on the network. Some general terms will be outlined, but the bulk of this chapter will be dedicated to the life of a QoS packet through a Catalyst 6500 switch.

QoS objectives are to provide reliable and available resources for applications and services on the network. In addition, QoS should offer some predictability of traffic flow seen on the network. Most networks today are implementing QoS for these very same reasons. Customers are looking for cheaper alternate means than simply increasing bandwidth on the network to support the various services they are supporting. However, bandwidth scarcity is not the only reason for QoS. Some applications are delay sensitive and require special handling on the network device itself.

QoS helps mitigate the following issues seen on the network:

  • End-to-end delay

    - Fixed delay

    - Variable delay (also known as jitter)

  • Bandwidth capacity issues

  • Packet loss