Core issue
When Quality of Service (QoS) is disabled, unicast traffic is assigned to queue 1. Broadcast, multicast, and unknown traffic is assigned to queue 2. If QoS is enabled, but the Class of Service (CoS)-to-transmit queue mappings are not modified, switch performance can be affected. This occurs because all traffic is assigned to queue 1.
Resolution
To correct the performance issues, modify the CoS-to-transmit queue mappings if QoS is enabled.
For more information, refer to Quality of Service on Catalyst 4500/4000 Switches Running CatOS FAQ.
For more information on Mapping CoS Values to Transmit Queues , refer to Mapping CoS Values to Transmit Queues and Drop Thresholds
