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Cat 4500 for VoIP call-signal markings discrepancy in Cisco documentation.

I have a Cat4506 with Sup V-10GE which is my core/distribution device to connect access layer 3750 switches to the WAN 2821 router.

One of the access 3750s has my PBX IP connections and I want to configure QoS for VoIP in this 4506 to trust the DSCP markings the PBX sets.

I've followed the Cisco Enterprise QOS SRND (3.3) which recommends these markings:

voice = dscp-46 PHB-EF

signalling = dscp-24 PHB-CS3

The 'Quality of Service on Cisco Catalyst 4500 Series Supervisor Engines' dated 2006, doesn't give any recommendations concerning dscp markings, but it does give the default behavior which is:

voice = dscp-46 PHB-EF

signalling = dscp-26 PHB-AF31

and 'auto qos voip trust' on the uplink ports creates these two lines:

qos map cos 3 to dscp 26

qos map cos 5 to dscp 46

Two questions:

1. Since I want to follow the Cisco Enterprise QOS SRND as far as dscp markings, is it just a matter of changing the .....

qos map cos 3 to dscp 26

qos map cos 3 to dscp 24?

2. Why is there a discrepancy between the 'Enterprise QOS SRND' recommendation and the 'QOS on 4500 Supervisor Engine' guides? Is this on purpose or an oversight do you think?


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Re: Cat 4500 for VoIP call-signal markings discrepancy in Cisco

It depends on the version of CallManager you have, or other IP PBX. With CCM 4.0 Cisco adopted IEEE standard to mark voice control as CS3 instead of AF31, however most IOS versions AutoQoS still remark it to AF31. If you have CCM 4.0+ I would suggest to map it to CS3 (24), but in your QoS class-map match both 24 and 26, ie:

class-map match-any VoIP_control

match ip dscp af31

match ip dscp cs3

HTH, please rate all posts!


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