From: Bill Eyer (beyer@optonline.net)
Date: Mon Sep 08 2008 - 19:58:37 ART
With this configuration you are only trusting COS, this would cause the
switch to re-write DCSP to 0. If you wanted to trust the DSCP value,
you would have to add mls qos trust dscp to the config. Alternately you
could rewrite the DSCP value to anything you like.
Sincerely,
Bill
Geert Nijs wrote:
> Hi Group,
>
> I have some particular QOS questions, especially regarding ingress and outgress interfaces.
> cos is a field present in dot1q links only.
>
> If an access port is configured with:
>
> switchport mode access
> switchport access vlan 103
> switchport voice vlan 203
> mls qos trust cos
>
> what happens if:
>
> - the port receives a frame without cos header, but with DSCP set to AF41 ??
> - is this considered cos 0 ? internal qos marking: DSCP 0 ?
> implied: no cos header, so default cos 0
> - is this considered cos 4 ? internal qos marking: DSCP CS4 ?
> implied: switch used dscp_to_cos map to derive cos from dscp
> and then used cos_to_dscp to get the internally used DSCP value.
> - is this considered dscp41 ? internal qos marking: DSCP AF41 ?
> implied: if no cos field is present, switch takes dscp field as is.
>
> If this packet is transmitted on the uplink and dscp rewrite is enabled (default setting), this packet
> will have a DSCP of 0 in the first case (loosing DSCP marking), DSCP CS4 (loosing drop probability) in the second case and DSCP AF41 in the third case (i presume no other policy-maps are present that recolor packets).
>
> regards,
> Geert
>
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