Cisco Catalyst 2000 - Voice Qos

254 Cisco LAN Switching Configuration Handbook
14-2: Voice QoS
To support proper delivery of voice traffic in a hierarchical switched network, follow sev-
eral QoS rules of thumb. See the basic network diagram in Figure 14-1.
■ Access layer
■ A QoS trust boundary should be established as close to the end devices (at the
access layer) as possible.
■ Let the IP Phone handle the trust boundary for attached PCs; the IP Phone should be
trusted.
■ PCs running Cisco SoftPhone should be untrusted. Instead, the inbound voice traffic
should be classified and the CoS and differentiated services code point (DSCP) val-
ues marked.
CallManager
Server Farm
Access Layer
L2: Trust CoS
L3: Trust CoS, DSCP
Access Laye
r
L2: Trust CoS
L3: Trust CoS, DSCP
L2: Don’t Trust CoS; Mark CoS=0
L3: Don’t Trust CoS, DSCP;
Mark CoS and DSCP=0
L2: Don’t Trust CoS; Mark CoS=0
L3: Don’t Trust CoS, DSCP;
Classify and
Mark Voice Packets
PC
PC
Cisco
IP SoftPhone
IP
Core or Distribution Layer
Cisco IP Phone
Figure 14-1 QoS Trust Considerations in a Switched Network