Re: mls qos cos override and the likes

In a simplified world, yes, this would be correct.

Reality differs in two ways:
1. Most of the time, the switch can understand only DSCP (therefore
requiring COS>DSCP and DSCP>COS maps)
2. The cos is present only when the output is trunk.

So there could be a situation, where the arriving COS is 5, you "trust cos",
map COS5 to DSCP20, and then map DSCP20 to COS4. (e.g. misconfig)
Depending if you have "rewrite DSCP" ON or OFF will determine if DSCP20 will
be written to the IP Header, or if the original DSCP will remain.
But if the output is trunk in this case, the COS will be set to 4 on the
output frame, even though you had "trust cos" on the input interface, and
the frame arrived with COS5.

HTH

On Sat, Sep 10, 2011 at 8:52 AM, imran ali <immrccie_at_gmail.com> wrote:

>
> so if a frame comes in with a cos value of 5
>
> interface fa0/1
> mls qos trust cos
> mls qos cos 3
>
> with the above config *...it retains cos of 5*
>
> the same frame with cos of 5 . when arrives on interface with below config
>
> interface fa0/1
> mls qos cos 3
> mls qos cos override
>
> *it get a new cos value of 3 .....*
> **
> **
> is this correct ?
>
>
>
> thanks
> On Fri, Sep 9, 2011 at 10:53 AM, Pavel Bykov <slidersv_at_gmail.com> wrote:
>
>> Emir, yes, like Adam described,
>>
>> the "mls qos cos" command is the "Fallback COS" or what marking should a
>> packet be assigned if the switch is confused and does not know what to do.
>> Examples of that is that you receive untagged traffic (access port, or
>> native vlan on a trunk port). In case the trust would be "mls qos trust
>> DSCP", the setting would also apply to all non-IP traffic, like IPX, LOOP,
>> DTP, VTP, CDP, STP, etc.
>> The "mls qos cos override" command is the highest priority command, that
>> makes everything behave according to the set cos, no matter the
>> trust/service-policy settings.
>>
>> The list of what takes precedence is the following:
>>
>> mls qos cos override
>> service policy
>> mls qos trust
>> mls qos cos
>> Pavel
>>
>> On Fri, Sep 9, 2011 at 12:16 AM, emir d souza <emir979_at_gmail.com> wrote:
>>
>> > Hi Guys,
>> >
>> > I'm a bit confused with the combination of commands for mls qos on an
>> > interface. The objective is to mark all incoming packets on that
>> interface
>> > with a cos marking of 3.
>> >
>> > What's the difference between
>> > interface fa0/1
>> > mls qos trust cos
>> > mls qos cos 3
>> >
>> >
>> > and
>> >
>> >
>> > interface fa0/1
>> > mls qos cos 3
>> > mls qos cos override
>> >
>> >
>> > and if you have both override and trust cos, which one will take
>> > precedence?
>> > Pls don't send me a Cisco link, the explanation from there is confusing.
>> >
>> >
>> > Cheers!
>> >
>> >
>> > Blogs and organic groups at http://www.ccie.net
>> >
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>> >
>> >
>> >
>> >
>> >
>>
>>
>> --
>> Pavel Bykov
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
>> _______________________________________________________________________
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>>
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>

-- 
Pavel Bykov
Blogs and organic groups at http://www.ccie.net