In today’s multiprotocol wide area networks (WANs), a diverse mixture of protocols and applications must share the same data path. When delay-sensitive applications run on these networks, the end user may see a severe degradation in the applications’ performance. For example, if an end user is attempting to teleconference across a WAN that is supporting many large file transfers, such as video and graphic files, SQL database transfers, and some authentication traffic thrown into the mix, the quality of the audio and video will suffer. If your WAN is supporting multiple protocols and time-sensitive applications, prioritization may be able to help.

What is prioritization?
Prioritization is a method of controlling bursty traffic on slow WAN links (those that run at 2.048 Mbps or slower). The key word here is bursty. If a WAN link is constantly congested, prioritization will not help much. In that case, it is best to look at increasing bandwidth to solve the problem.

Essentially, prioritization assigns a priority level to each type of traffic on the WAN and allows the higher priority traffic to cross the WAN interface before the lower priority traffic. Cisco implements prioritization using one of four methods:

  • First in, first out (FIFO) queuing
  • Weighted fair queuing
  • Priority queuing
  • Custom queuing

Cisco’s queuing methods
The first queuing method used by Cisco devices is FIFO. First in, first out queuing does exactly what its name implies. The first packet received on a WAN interface is the first packet sent out of the WAN interface without regard for bandwidth utilization or application time-sensitivity. While this method is sufficient for supporting high-speed WAN links (those operating above 2.048 Mbps), it does not help much with time-sensitive traffic running on slower WAN links.

Weighted fair queuing is useful in environments that support low volume traffic. For example, in a network running Telnet and file transfer protocol (FTP), weighted fair queuing would assign the highest priority to Telnet. The weighted fair queuing algorithm keeps track of low volume traffic based on conversations. Weighted fair queuing then ensures that the low volume conversation receives priority over other types of traffic. For example, a Telnet session would receive priority over a large file download that might otherwise consume the entire bandwidth of the WAN link.

Priority queuing is a method of administratively assigning priority levels to each type of traffic that crosses the WAN link. An administrator creates a priority list and assigns a specific priority level to each protocol type: high, medium, normal, or low. For example, in a network where FTP traffic has the highest importance, FTP would be assigned to the high priority queue and other types of traffic would be assigned to the medium, normal, or low queue. When traffic arrives on a WAN interface, it is assigned to the appropriate queue, and the traffic is forwarded based on that queue. Caution: This approach can starve some traffic of bandwidth because all traffic in the high queue must be processed before traffic in the medium queue, and so on. If there is a lot of traffic in the high queue, the traffic in the medium, normal, and low queues may time-out and be dropped before they can be processed.

The last queuing method used by Cisco is custom queuing, which allows the network administrator to assign traffic to one of 16 queues and then allocate bandwidth to each queue. Custom queuing can be very difficult to configure and manage, but it does give the network administrator more granular control over network traffic.

Want more?
Prioritizing network traffic using one of the four Cisco queuing methods can dramatically increase application response time and improve traffic control over slow WAN links.

For more information on configuring prioritization on your Cisco devices, check out ACRC Exam Certification Guide by Cisco press.

Warren Heaton CCDA, CCNA, MCSE+I is the Cisco program manager for A Technological Advantage in Louisville, KY.

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