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Q. My server cannot tag Class of Service (CoS) values. Can the Catalyst
4500/4000 switches that run CatOS tag the traffic from the server for a
specific CoS value?
A. Yes, but remember that the default CoS for untagged packets is switch−wide and not on a
per−port basis. So, all untagged packets are marked the same CoS value. Catalyst 4500/4000
switches that run Cisco IOS Software do support tagging on a per−port basis. For more
information, refer to Understanding and Configuring QoS.
Q. Do the Catalyst 4500/4000 switches that run CatOS honor the
incoming Class of Service (CoS) value in IEEE 802.1Q (dot1q) tags from
my IP phones?
A. Yes, Catalyst 4500/4000 switches that run CatOS do honor the incoming CoS value in the
dot1q tag. Since dot1q does not tag the native VLAN, you need to use the switch−wide CoS
configuration to tag such packets. These tags are retained through the switch and used in
output scheduling. If the outgoing port is a trunk, the original CoS value or the new value (for
packets that arrive untagged on native VLAN) is tagged in the packet.
Q. Can the Catalyst 4500/4000 switches that run CatOS extend the trust
or override the incoming Class of Service (CoS) value of devices
connected to the IP phones?
A. No, Catalyst 4500/4000 switches that run CatOS cannot extend the trust or override the
incoming CoS value of traffic from devices connected to the IP phones. Catalyst 4500/4000
switches that run Cisco IOS Software can support extended trust. Refer to the document
Configuring Voice Interfaces.
Q. What kind of output scheduling do the Catalyst 4500/4000 switches
that run CatOS provide?
A. Line cards of Catalyst 4500/4000 switches that run CatOS support two output queues per
port with one threshold at 100 percent. This 2 Queues, 1 Threshold (2Q1T) method is not
configurable. It does offer user configuration for Class of Service (CoS) value maps for the
two queues. For example, you can configure packets with a CoS value of 0› to use the first
queue and 4Ÿ to use the second queue. Catalyst 4500/4000 only supports CoS mappings in
pairs, 0™, 2›, 4, 6Ÿ. You cannot set one CoS value unless you specify the
corresponding partner CoS. For example, you cannot specify 0œ for the first queue because
you must pair 5 with the partner, 4. The two queues are serviced in a round−robin fashion.
Refer to the Installation and Configuration Note for the Catalyst 4000 Layer 3 Services
Module for additional information on Layer 3 module features. For Catalyst 4500/4000
switches that run Cisco IOS Software features, refer to the document Understanding and
Configuring QoS.
Q. I enabled QoS on a Catalyst 4500/4000 switch that runs CatOS and
now I see performance issues. What is wrong?
A. When QoS is disabled, unicast traffic is assigned to queue 1 and broadcast, multicast, and
unknown traffic is assigned to queue 2. If you enable QoS but do not modify the Class of
Service (CoS)−to−transmit queue mappings, switch performance can be affected because all
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