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Qos Cisco Voice - Help

Posted on 2007-04-03
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Last Modified: 2010-04-17
Hi Experts ...
I need your help again ...

I have a network connected to the Internet through a Cisco 1721 attached to a 1024 Kbps link from my provider. This network is a little congested now and the most important thing is voice so I need strict priority on this. So ... My current config is ...

LAN port -> FastEthernet0 ------------ WAN port -> Serial0

As the packets enter the router I mark them as VOICE dscp 46 and OTHER TRAFFIC as best-effort (dscp 0), I do this through access-lists coming and going to my PBX box.
At the Internet Edge, I configured LLQ for the packets going out giving the strict priority to the VOICE traffic. And for the packets coming from the Intenet, I rate-limited them and marked with precedence 5 ...

Everything was going ok ... but now I'm experiencing some dead spots on voice ... The latency from my network to my providers is not so low, so I can't add more latency inside the network ...

I checked both the policy maps and rate-limits regarding the Voice traffic and I haven't any packet exceeded ... So I can just think about the MTU that I left default (1500) because as I tried to change in the beginning I had some issues with other applications, including SIP traffic leaving the network ...

I'd like to implement LFI, but I don't know if I can implement in my Serial interface ... Is that possible ? Is there any way to make the packets smaller so they do not add more latency on the edge of the network ? Is there anything that I'm missing with this configuration ?

Thanks,
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Question by:marcelocbf
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mikebernhardt earned 1000 total points
ID: 18853534
Well, one problem is that although you are marking you inbound packets when they reach you, you have no control over the priority of those packets as they drop onto the T1 toward you. If your ISP is willing to do some QOS for you, that's great. Otherwise you can only do so much.
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by:marcelocbf
ID: 18854348
Yeah ... Unfortunately we cannot control everything ... hehe
But I think that if I fragment the packets and interchange (LFI) them with some priority is gonna help, but I don't know if I can do this on my Serial Interface ...

But do you think is there any other way to do that ? Because changing the MTU is a headache for me ... and I know it would just solve the outbound audio ...

Thanks anyway,
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