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QOS question posted 09/08/2008
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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).


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