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Cisco® Field Manual: Catalyst® Switch Configuration by Steve McQuerry, Dave Hucaby

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14-2. Voice QoS

To support proper delivery of voice traffic in a hierarchical switched network, follow several 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) values marked.

    - Normal PCs with no voice capability should be untrusted (CoS and type of service [ToS] set to 0).

    - On Catalyst 6000 switches, port trust can be VLAN-based and applied to the voice VLAN ...

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