Cisco Catalyst QoS Trust Concept 61
Catalyst QoS Congestion Management
Catalyst switches use scheduling and transmit queues to achieve congestion management.
All the currently shipping switches in the Catalyst product line support a form of sched-
uling that is more advanced than FIFO. The specific mechanisms differ on a platform basis.
With regard to output scheduling, the QoS marking determines the scheduling and output
queue. In the case of the Catalyst 6500 Family of switches, for example, there are different
queue types and drop thresholds per line module. Other platforms, such as the Catalyst
3500 Family and the Catalyst 4000 IOS Family of switches, use a single queue type for all
line modules and product families.
Congestion Avoidance
At time of publication, only the Catalyst 3550 Family and the Catalyst 6500 Family of
switches support congestion avoidance. In brief, congestion avoidance attempts to prevent
congestion by applying specific queuing parameters. The Catalyst 3550 Family and the
Catalyst 6500 Family of switches utilize WRED and several other queuing configurations
to support congestion avoidance. This book discusses the Catalyst 3500 Family and
Catalyst 6500 Family of switches in Chapters 6 and 8, respectively.
Cisco Catalyst QoS Trust Concept
The trust concept is a classification configuration option supported on all Catalyst switches
that support QoS classification. The trust state of a switch port or interface defines how
ingress packets are classified, marked, and subsequently scheduled. For a Cisco Catalyst
switch that bases QoS only on CoS values, a port that is configured as untrusted reclassifies
any CoS values to zero or to a statically configured CoS value. The CoS values of packets
arriving on an untrusted port are assumed not verifiable and deemed unnecessary by the
system administrator of the switch. Depending on the platform, untrusted ports may be
configured to reclassify or mark IP precedence, DSCP, or CoS values on any ingress frame
based on an 802.1q tag or access list.
Figure 2-8 illustrates the QoS trust concept. A workstation attached to a Catalyst 6500
switch is sending 802.1q tagged frames to the Catalyst 6500 switch with a CoS value of 5.
If the port is configured as untrusted, the switch sets an internal DSCP value associated with
the frame to 0. The switch does not actually alter the CoS value of the frame until trans-
mission. All untrusted ports set the internal DSCP to 0 by default. However, the overriding
internal DSCP value is configurable on various platforms. If the switch port is configured
for Trust-CoS, the CoS value is not altered on ingress. Figure 2-6 applies to trusting IP
precedence and DSCP values as well.

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