mls qos cos override

[ciscoforum]

trying test override option on 3550,but it seems the result is not same as document says. An ip phone is on port 0/15 where there is no pc behind it. There is no any call yet. I just want to see how the override works. According to document this option will remark all the tagged and untagged packets's dscp value to the default setting, this case 5. But the following output shows that dscp 26(sccp)still not changed to dscp 46, only dscp 0 (no traffic on this entry because it has been listed on dscp46 already) has been changed to 46. Just FYI in the other test I have done , if there is no any qos setting on the interface I noticed tftp/dhcp and some other traffic doesn't have tag and belongs to dscp 0 entry. So the override still only applies to the untagged packet, not as document says apply to both tagged and untagged packet.

3550-24#sh run | include mls qos

mls qos map cos-dscp 0 8 16 26 32 46 48 56

3550-24#sh run inter fas0/15

Building configuration...

Current configuration : 295 bytes

!

interface FastEthernet0/15

switchport trunk encapsulation dot1q

switchport trunk native vlan 101

switchport mode trunk

switchport voice vlan 201

no ip address

mls qos cos 5

mls qos cos override

mls qos trust cos

mls qos monitor dscp 0 8 24 26 32 40 46 48

spanning-tree portfast

end

3550-24#sh mls qos inter fas 0/15 sta

FastEthernet0/15

Ingress

dscp: incoming no_change classified policed dropped (in bytes)

0 : 0 0 0 0 0

8 : 0 0 0 0 0

24: 0 0 0 0 0

26: 472 472 0 0 0

32: 0 0 0 0 0

40: 0 0 0 0 0

46: 268 268 0 0 0

48: 0 0 0 0 0

Others: 0 0 0 0 0

Any idea, thanks?

[dwyerr]

Hello.

I think I have an idea of what you are asking,if I have missed the question let me apologize up front:)

The "mls qos cos override" command overrides COS not DSCP. (Remember, COS = layer 2 & DSCP = layer 3. It gets a little bit hairy hear because the switch also uses DSCP values internally.) In this specific example the "override" knob overrides all frames, tagging them with a new COS value and the "cos-dscp map" maps the new COS value (layer 2) to a DSCP value that will be used internally by the switch. This is the function of the cos-dscp map.

So in your case you have marked all tagged and untagged frames (layer 2) with a COS value of 5. When this newly marked frame leaves the incoming port and goes into the switch fabric it gets marked with a DSCP value of 46. Now if the frame is going to another layer 2 port it will go back through the cos-dscp map and get the appropriate value. If it is leaving the switch via IP the frame will get wrapped in an IP packet and the DSCP value should be 0, the default.

HTH.

-Rob

[ciscoforum]

Thanks for your answer.But I think DSCP26 should be same as dscp0 in my case and should have no packets at all.all the DSCP26 packets should be under DSCP46 entry like DSCP0 because they are all marked as COS5 and in turn layer 3 is 46.I believe the packets under DSCP46 you see now is the packets supposed to be DSCP0 before because as I said in the other test I do see DSCP0 packets if I don't have override option.This is what confuse me.