User`s guide
216 XSR User’s Guide
Mechanisms to Provide QoS Chapter 10
Configuring Quality of Service
Configuring Priority Queues
The
priority command configures priority queuing for certain packets
based on the traffic class. When you specify priority (using the following
commands) for a class, it takes a bandwidth argument affording maximum
bandwidth. The following commands configure priority queuing:
policy-map policy-name
class class-name
priority priority-level kbps [burst-size]
Be aware that bandwidth guarantees come into play when an interface is
congested, at which time traffic class guarantees bandwidth equal to the
specified rate. The
priority command implements a maximum bandwidth
guarantee. If the priority class does not use its bandwidth, the excess
bandwidth may be used by CBWFQ. A rule of thumb for configuring PQs is
to assign time-sensitive traffic (voice and video) to PQs and other types (e.g.,
Telnet) to fair queues. Any traffic you do not specially assign (e.g., Email) is
automatically directed to the class-default queue. All (100%) of your traffic
should not be assigned to PQs - a smaller percentage of lower priority traffic
should be designated for fair queues of left unassigned for the default queue.
Internally, the priority queue uses a Token Bucket that measures the offered
load and ensures that the traffic stream conforms to the configured rate. Only
traffic that conforms to the token bucket is guaranteed low latency. Any
excess traffic is dropped even when the link is not congested.
The
priority command also sets burst size, a network value used to
accommodate temporary bursts of traffic. The default burst value, which is
computed as 1 second of traffic at the configured bandwidth rate, is used when
the burst argument is not specified.
The XSR allows the priority queue size to grow as much as allowed by the
traffic meter.
The following example illustrates priority configuration options and how
they are invoked on a Frame Relay port. Begin by creating traffic class frost:
XSR(config)#class-map frost
XSR(config-cmap<frost>)#match access-group 10