mls qos trust

[fran19422]

Hello, if the command 'mls qos trust xxxxx' is not issued, and qos is turned on for the interface, does this mean the switch will erase all cos and dscp markings received, therefore preventing me from testing packets/frames against these cos/dscp values ?

So if I want to set up class maps, policy maps, and then service policies, it is essential that I:

1. turn on mls qos ?

2. enter a trust statement in order to preserve the cos or dscp values that I want to test against ?

3. now I can test against against cos or dscp values ?

Thanks for clarification.

[Dennis Mink]

That is correct, when you would use for instance mls qos trust cos. You would need to define you cos<>dscp mappings on the switch and the switch will apply qos accordingly.

So really if you have an ingress switch port and you trust cos or dscp, you can still have egress policies on a port (on the same switch), using these cos or dscp values.

the mls qos trus command is just a way to make it easier to rely on existing cos/dscp values that a phone sends (based on your CUCM configuration,), without the need for you having to configure it explicitly on each access port.

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[fran19422]

thanks for that.

But even if I do use the command 'mls trust xxxx', the switch will still nor perform any QoS until I enter the command 'auto QoS' or manually configure some service policies - correct ?

[Dennis Mink]

Correct,

You have to remember that once the DSCP/Cos settings are done on an ingress port (be it through auto Qos or mls cos trust) , you need to do someting with these markings, this is done through class maps>policy maps>service policy.  so really it doesnt matter where the values come from as long as they are moulded into a class map.

check this 3 minute tutorial on MQC:

http://www.youtube.com/watch?v=lzPA6Q5T7Ng

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