Configuring an ATM PVC to trust DSCP

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Sep 4th, 2007
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This sounds like an easy one, but I'm struggling with the command syntax to allow an ATM PVC to trust incoming DSCP values marked downstream. Using 3745 routers with 12.3T code, it appears that mls rp ip needs to be enabled but when I try to do this on a sub-interface/PVC, it wants a VLAN-ID. I'm a bit confused and wouldn't have thought it would be this difficult to implement. Anyone any ideas?


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Joseph W. Doherty Tue, 09/04/2007 - 15:45
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When you write you need to trust inbound DSCP markings, trust how? Reason I ask, don't recall Cisco routers normally remarking an inbound DSCP unless you configure it to do so.

smith.dean Tue, 09/04/2007 - 15:51
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You're right. I think that was the concept I was struggling to grasp; whether or not you need to explicitly configure the WAN interface as a trust boundary or not. Or to put it another way, will it re-write values by default or not. I wasn't sure and the fact i couldn't find any doc on CCO for how to configure an explicit trust probably should have alerted me to the fact you don't have to. I just labbed this up as well and sure enough, it will just pass the DSCP values by default (unless, as you suggested, you specifically apply a service-policy to change it). Thanks for your help though, it is much appreciated.

Joseph W. Doherty Tue, 09/04/2007 - 16:25
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Great! Be warned though, believe some of Cisco switches do reset DSCP unless you explicitly set them to "trust".

smith.dean Tue, 09/04/2007 - 17:06
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Yeah, switches do for sure. That's what led to my confusion with the router platforms. I've explicitly set the trust interfaces on the switches as part of the mls qos configurations and I'm comfortable with that. Now I know the routers will trust by default, I'm find with that too. thanks for your interest though Joseph.


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