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Newb VoIP End to End Qos Questions

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Mar 31st, 2010
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I'm at a site that has Layer 3 Cisco switches and routers end to end. Cisco IP phones plug directly into Layer 3 switches with various routers in the core.

1) If all connections are Layer 3 do I need to concern myself with trusing COS? I would trust DSCP in this case yes?

2) Cisco phones by default send voice rtp audio with DSCP marked as EF, is that correct? If not, I know how to create policy-maps and reclassify it. My thought was it is already classified as high as possible.

3) I've read that QoS must be end to end or you don't really have a full implementation, as in the one device your traffic passes through that is not performing QoS will likely be running FIFO or WFQ, which do not give precedence to class based traffic. So in theory I need to implement CBWFQ or LLQ on every Layer 3 device end to end that RTP audio will touch. Do I have this correct?

4) If the voice traffic has the potential to travel to most Layer 3 devices in the network as it travels between the Call Manager and phones, I need to apply QoS outbound on each Layer 3 interface that is likely to be in a voice path yes?

Sorry, I'm a newb, I understand it in theory but I still hit the wall on occassion with my light saber while blindfolded.

Thanks, Mike

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Aaron Harrison Wed, 03/31/2010 - 14:28
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1) Yep - no trunk (dot1q/ISL) = no CoS as it's in the trunk header, so use DSCP.

2) Yep, traffic will be marked. You can trust it, or trust it conditionally in a service policy and police it.

3) Yep, more or less. It's very important that on any switches that have QoS enabled, you have proper trust states set on all the ports, otherwise the DSCP will be remarked to 0. This is worse that not particularly prioritizing voice on a particular device, as no other devices further down the chain will see any DSCP markings to act on.

4) Erm.. kind of. If it's a layer 3 link on a switch, then you don't configure it as you would on a router. The QoS is done in hardware, so your common/garden autoqos type config should see you right if you really don't know what you are doing. If you use auto qos voip trust, make sure you set it to mls qos trust dscp after as it will probably default to CoS. Best to keep it consistent...



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michael.perry Thu, 04/01/2010 - 14:12
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Muchos gracias. I'll give it a go and test it thoroughly for the desired results.



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