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Chapter 3 QoS in an AVVID-Enabled Campus Network
Selecting an Access-Layer Switch
Figure 3-6
General Model for Catalyst 6500 QoS Configurations
With the addition of the PFC, the Catalyst 6500 is capable of handling Layer 2, 3, and 4 QoS tasks. The
PFC can be used to enable advanced QoS tools, such as packet classification and marking, scheduling,
and congestion avoidance, based on either Layer 2 or Layer 3 and 4 header information. You can
configure multiple receive and transmit queues with thresholds that can be used according to the QoS
policy rules configured in the switch.
There are also many versions of Catalyst 6500 line cards with varying QoS capabilities. (See
Appendix A, "Reference Information"
for a list of linecards and queuing mechanisms available for the
Catalyst 6500.) The newest cards include enhanced QoS features that provide priority queuing
capabilities for both ingress and egress interfaces. With the exception of the priority queue, which is
always serviced as soon as frames are present, the queues are serviced in the WRR method. Each queue
is given a user-configurable weight. By default, the "high" queue is given 98% of the scheduler time and
the "low" queue is given 2%. This ratio is conducive to ensuring that packets with a low delay-tolerance
are not delayed in a queue. This is also the reason behind giving the "low" queue a much higher
percentage of the overall interface buffer.
Note
Because the Catalyst OS is designed for switches that operate in the "closet" and Native IOS is designed
for switches that operate at the distribution layer of the network, a Catalyst 6500 that is functioning as
an access-switch should be equipped with Catalyst OS.
To see how a port is configured, issue the show port capabilities mod/port Catalyst OS command. The
default QoS capabilities of the port can be changed using the set qos map and set qos wred-threshold
commands. When modifying the queue thresholds, it is important to remember that the higher priority
queue has a higher numerical value.
Si
Si
Si
IP
VVID=110
VLAN=10
Core
Distribution
Access
Catalyst
6500
Catalyst 6500 w/ PFC
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