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Friday, March 11, 2011

Once a frame has entered a switch port where a phone is plugged in, does the cos get set automatically according to the dscp - cos map?

We are setting up our first voip network and are using Autoqos. We have a couple of questions and hope an experienced engineer can confirm our assumptions.We are using Shoretel phones which are advised set DSCP "ef" in the TOS byte. Apparently they don't set cos. This means we can trust the phone to mark the packets, so I use this command to configure auto qos:

"auto qos voip trust"

normally auto qos sets:

"mls qos trust cos"

But we assume that because my phone set DSCP we should change this to:

"mls qos trust dscp"

Does this sound correct so far?

Next quesiton is:

Once a frame has entered a switch port where a phone is plugged in, does the cos get set automatically according to the dscp - cos map?

If so our assumption is that all switch 802.1q trunk/uplinks We can leave as the default:

"mls qos trust cos".

Tips

You'll want to change your switch to trust DSCP.Once the frame comes into the switch and you have DSCP trusted, when it egresses a trunk we will use the DSCP -> Cos mapping table to mark COS. Also keep in mind that the 'trust' statement only applies to ingress traffic on that port. Not that it really matters here in your example, just stating that regardless of your egress trunk link trusting cos or dscp...it won't change the fact that we will still do a DSCP->COS mapping and mark the traffic. Its whats on the other side that matters, it will determine whether or not to trust dscp vs cos.

One think to keep in mind is that traffic egressing a trunk on the native vlan will not have an 802.1q tag, so therefor won't have a COS value. If the receiving end is trusting COS, we interpret this as the packet having COS o. Just something to keep in mind. You can also set your trunks to trust DSCP if you may have traffic that needs to be marked crossing your native vlan.

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