Specifications

10-15
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Chapter 10 Traffic Shaping
Low-Latency Queueing
can give one or more classes priority status. When multiple classes within a single
policy map are configured as priority classes, all traffic from these classes is
enqueued to the same single, strict priority queue.
One of the ways in which the strict PQ used within CBWFQ differs from its use
outside CBWFQ is in the parameters it takes. Outside CBWFQ, you can use the
ip rtp priority command to specify the range of UDP ports whose voice traffic
flows are given priority service.
Using the priority command, you are no longer limited to a UDP port number to
stipulate priority flows because you can configure the priority status for a class
within CBWFQ. Instead, all of the valid match criteria used to specify traffic for
a class now apply to priority traffic. These methods of specifying traffic for a class
include matching on access lists, protocols, and input interfaces. Moreover,
within an access, list you can specify that traffic matches are allowed based on the
IP differentiated services code point (DSCP) value. This value is set using the first
six bits of the ToS byte in the IP header.
Although it is possible to enqueue various types of real-time traffic to the strict
priority queue, Cisco strongly recommends that you direct only voice traffic to
this queue. The reason is that voice traffic is well-behaved, whereas other types
of real-time traffic are not well-behaved. Moreover, voice traffic requires
nonvariable delays to avoid jitter.
Real-time traffic such as video could introduce variation in delay, thereby
thwarting the steadiness of delay required for successful voice traffic
transmission.
For more conceptual information about LLQ, see the section “Weighted Fair
Queueing” in the Cisco IOS Quality of Service Solutions Configuration Guide,
Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
_c/fqcprt2/qcfconmg.htm#xtocid46014
For information on how to configure LLQ, see the chapter "Configuring Weighted
Fair Queueing" in the Cisco IOS Quality of Service Solutions Configuration
Guide, Release 12.2:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
_c/fqcprt2/qcfwfq.htm