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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 38 Configuring Standard QoS
Information About Standard QoS
For most Cisco IP phone configurations, the traffic sent from the telephone to the switch should be
trusted to ensure that voice traffic is properly prioritized over other types of traffic in the network. By
using the mls qos trust cos interface configuration command, you configure the switch port to w hich
the telephone is connected to trust the CoS labels of all traffic received on that port. Use the mls qos
trust dscp interface configuration command to configure a routed port to which the telephone is
connected to trust the DSCP labels of all traffic received on that port.
With the trusted setting, you also can use the trusted boundary feature t o prevent misuse of a
high-priority queue if a user bypasses the telephone and connects the PC directly to the switch. Without
trusted boundary, the CoS labels generated by the PC are trusted by the switch (because of the tru sted
CoS setting). By contrast, trusted boundary uses CDP to detect the presence of a Cisco IP phone (such
as the Cisco IP phone 7910, 7935, 7940, and 7960) on a switch port. If the telephone is not detected, the
trusted boundary feature disables the trusted setting on the switch port and prevents misuse of a
high-priority queue. Note that the trusted boundary feature is not effective if the PC and Cisco IP p hone
are connected to a hub that is connected to the switch.
In some situations, you can prevent a PC connected to the Cisco IP phone from taking advantage of a
high-priority data queue. You can use the switchport priority extend cos interface configuration
command to configure the telephone through the switch CLI to override the priority of the traffic
received from the PC.
DSCP Transparency Mode
The switch supports the DSCP transparency feature. It affects only the DSCP field of a packet at egress.
By default, DSCP transparency is disabled. The switch modifies the DSCP field in an incoming packet,
and the DSCP field in the outgoing packet is based on the qua lity of service (QoS) configuration,
including the port trust setting, policing and marking, and the DSCP-to-DSCP mutation map.
If DSCP transparency is enabled by using th e no mls qos rewrite ip dscp command, the switch does not
modify the DSCP field in the incoming packet, and the DSCP field in the outg oing packet is the same as
that in the incoming packet.
Note Enabling DSCP transparency does not affect the port trust settings on IEEE 802.1Q tunneling ports.
Regardless of the DSCP transparency configuration, the switch modifies the internal DSCP value of the
packet, which the switch uses to generate a class of service (CoS) value that represents the priority of
the traffic. The switch also uses the internal DSCP value to select an egress queue and threshold.
DSCP Trust State on a Port Bordering Another QoS Domain
If you are administering two separate QoS domains between which y ou want to implement QoS features
for IP traffic, you can configure the switch ports bordering the domains to a DSCP-trusted st ate as shown
in Figure 38-12. Then the receiving port accepts the DSCP-trusted value and avoids the classification
stage of QoS. If the two domains use different DSCP values, you can configure the
DSCP-to-DSCP-mutation map to translate a set of DSCP values to match the definition in the other
domain.