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QoS Questions

I am a semi newbie at a lot of this. I am starting to work on configuring QoS across my LAN/WAN environment. This is being done because I have VoIP trunks connecting phone switches at various locations back to my main phone switch. I am just trying to make sure that I am on the right track regarding my initial thoughts on QoS. I realize there are many ways that this can be configured so any/all thoughts are welcome. My thought is to use extended acl's on my cisco routers and switches by using the ip address of the phone switches setting ToS to 5. This should get the VoIP traffic on all of the local LANs prioritized to the edge. Then across my WAN connections I was thinking of using NBAR and then prioritizing all of my RTP traffic using set precedence 5. I have firewalls between routers and switches and wonder if they can prioritize packets based on this ToS setting as well?

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Re: QoS Questions

The prioritization is going to depend on the queues that you have set. The best way of doing things would be to mark as close to the edge of the network as possible, the queue it appropriately. At the access layer, you would want to mark Voice traffic with a CoS value (layer two value) 5 and signaling traffic with a CoS value of 3 (this is automatically done by cisco phones. Then it is a question of what kind of switch that you have. If you have a 2950, the mapping of the CoS value to the DSCP value (layer 2 to layer 3) does not follow the standard. CoS 5 will map to DSCP 40 (which should be DSCP 46 for voice, according to best practice). To correct this, you could create a mutation map to handle this. If you have a 3550 or higher, then you can remap the cos values to the dscp values globally on the switch by using the following:

mls qos map cos-dscp 0 8 16 24 32 46 48 56

After marked at layer two and queued properly, further up the stream is not as complicated. Hope this helps.

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