Delaysensitive applications may require higher priority than others

© 2004 Cisco Systems, Inc. All rights re bcran v2.1—10-15

Complete these steps when you are choosing a Cisco IOS queuing option:

Step 1 Determine whether the WAN is congested.

If traffic does not back up, there is no need to prioritize it. The traffic is serviced as it arrives. However, if the load exceeds the transmission capacity for periods of time, you may want to prioritize the traffic with one of the Cisco IOS queuing options.

Step 2 Decide whether strict control over traffic prioritization is necessary and whether automatic configuration is acceptable.

Proper queuing configuration is a nontrivial task. The network manager must study the traffic types traversing the interface, determine how to classify them, and decide on their relative priority. The manager must install the filters and test their effect on the traffic. Traffic patterns change over time, so the analysis must be repeated periodically.

Step 3 Establish a queuing policy.

A queuing policy results from the analysis of traffic patterns and the determination of relative traffic priorities discussed in Step 2.

Step 4 Determine whether any of the traffic types identified in your traffic pattern analysis can tolerate a delay. Typically, voice and video have the lowest tolerance for delay.

The table illustrates the typical queuing options a network administrator would choose from when determining how to best implement a queuing policy.

Queuing Options

Queuing Type

Description

FIFO

FIFO queuing is simply sending packets out of an interface in the order in which they arrived.

PQ

Priority queuing (PQ) defines four priorities of traffic—high, normal, medium, and low—on a given interface. As traffic comes into the router, it is assigned to one of the four output queues. Packets on the highest-priority queue are transmitted first; packets on the next highest-priority queue are transmitted second; and so on.

CQ

Custom queuing (CQ) reserves a percentage of bandwidth for specified protocols. Up to 16 output queues can be configured for normal data and an additional queue can be created for system messages such as LAN keepalives. Each queue is serviced sequentially, by transmitting a configurable percentage of traffic and then moving on to the next queue.

WFQ

WFQ provides traffic management that dynamically prioritizes traffic into conversations, or flows, based on Layer 3 or 4 information. It then breaks up a stream of packets within each conversation to ensure that bandwidth is shared equally between individual conversations.

CBWFQ

CBWFQ defines traffic classes, typically using access control lists (ACLs), and then applies parameters, such as bandwidth and queue-limits, to these classes. The bandwidth assigned to a class is used to calculate the "weight" of that class. The weight of each packet that matches the class criteria is also calculated. WFQ is then applied to the classes, which can include several flows, rather than to the flows themselves.

LLQ

LLQ provides strict PQ for CBWFQ, reducing jitter in voice conversations. Strict PQ gives delay-sensitive data, such as voice, preferential treatment over other traffic. With this feature, delay-sensitive data is sent first, before packets in other queues are treated. Low latency queuing is also called PQ/CBWFQ because it is a combination of the two techniques.

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