fisrt for ur knowledge

trust always based on two layers L2 with COS and layer three with with IP precedence or DSCP

on access port that connect to ip phones for example make the trust on L2 on the uplink fron access to distribution layer and on the distribution uplink as well make it lyaer three trust dscp because if u keep it on layer two and the packet gos from layer two to three device it will loss its marking so make it on unlinks as lyaer three trust to keep ur marking end-to-end

once u trust the cos marking for exampe cisco ip phones bydefault mark the voice traffic with cos 5 and voice signaling traffic with cos 3 then u can match this traffic with a class map and give it spesific bandwidth through a policy-map

and the following link nice source

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml

good luck

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