QOS Trust priority on a switch port Question

[ROBERT T]

Can somebody help me understand what is trusted in the below config and find the documentation to back it up?

There are 3 QOS statements here, "mls qos trust DSCP", "mls qos trust device cisco-phone" and "auto qos voip trust".

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Alone, mls qos trust dscp will trust any dscp markings coming in.

Alone, mls qos trust device cisco-phone will only trust packets from a cisco IP phone.

Alone, auto qos voip trust will trust the CoS value for nonrouted ports or the DSCP value for routed ports in ingress packets 

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What happens when I hook up a NON-Cisco IP phone? Is anything trusted if the NON-Cisco phone does not talk CDP? What about a soft Phone on the computer?

If I remember correctly, "mls qos trust device cisco-phone" trumps "mls qos trust dscp" and only a Cisco IP phone would be trusted. Is this correct?

interface FastEthernet1/0/2    (Cisco 3750 - 12.2(50))
 description LOCAL LAN
 switchport mode access
 switchport voice vlan 20
 srr-queue bandwidth share 10 10 60 20
 srr-queue bandwidth shape  10  0  0  0
 mls qos trust device cisco-phone
 mls qos trust dscp
 no snmp trap link-status
 auto qos voip trust

[Mark Malone]

yes a lot of trusting going on there , when you enable auto qos voip trust it actually enables mls qos trust dscp , which means when the phones marks the rtp packet with EF 46 DSCP the local port will trust that marking and carry it forward as qos prefers markings closest to the source , you would then have mls qos trust dscp on your uplink as well to continue to carry the marking through the network  , you would only use trust device cisco phone if it is an actual cisco phone , dont use the cisco phone command if its like avaya or something else  , the fact though you have mls qos trust dscp there as well it will still trust other phones , the cisco phone trust is only for hardware so if you have a softphone use auto qos voip trust or mls qos trust dscp on the access port

The best way to check this is span the port you will see the markings in the packet as the device sends them