Re: Layer 2 qos

Adam,

Thank you and I appreciate your help very much...

---- Adam Booth <adam.booth_at_gmail.com> wrote:
> Hi,
>
> To set the 802.1P value (Layer 2 CoS) value, you need to be using 802.1Q -
> this is not possible on an access port (though if you are using a Cisco IP
> phone, trunking will happen transparently using the Voice-VLAN feature if
> configured but lets not go there)
>
> 3550s use the concept of an "internal DSCP" value for all frames that enter
> the switch - even 802.1P CoS values get mapped to an internal DSCP which you
> can see if you execute "sh mls qos maps cos-dscp"
> By default ports are considered untrusted and therefore given an internal
> DSCP of 0.
> Upon egress from the switch, traffic will be placed into egress queues based
> on the internal DSCP value
> On the 3550, if you are carrying IP packets within those Layer 2 Frames you
> will have the original IP DSCP value over-written with the internal DSCP
> value
> If you are egressing on a 802.1Q trunk, the 802.1P CoS value for the frame
> will be written based on a dscp-cos mutation map.
>
> MQC would probably the be the best tool for what you want to do if you don't
> trust the device coming in to have the correct DSCP value set - you can
> force everything to have a particular DSCP value
>
> Cat3550-3#sh mls qos maps dscp-cos
> Dscp-cos map:
> d1 : d2 0 1 2 3 4 5 6 7 8 9
> ---------------------------------------
> 0 : 00 00 00 00 00 00 00 00 01 01
> 1 : 01 01 01 01 01 01 02 02 02 02
> 2 : 02 02 02 02 03 03 03 03 03 03
> 3 : 03 03 04 04 04 04 04 04 04 04
> 4 : 05 05 05 05 05 05 05 05 06 06
> 5 : 06 06 06 06 06 06 07 07 07 07
> 6 : 07 07 07 07
>
>
> Looking at this, we can see internal DSCP values 40-47 match cos value 5 (I
> would probably pick DSCP 40 to use as the cos-dscp map associated cos value
> 5 with internal DSCP 40)
>
> If you classify incoming traffic to be one of those DSCP values, it should
> leave the switch on a trunk with a cos value of 5 (since the 3550 doesn't
> support "no mls qos rewrite ip dscp" like 3560s do, the payload IP DSCP
> values will be overwritten with the internal DSCP value)
>
> Remember when you are playing with qos on the switches, you need to enable
> qos (mls qos) and unlike a router, show policy-map isn't actually going to
> give you a useful output - so don't pull you hair out if things don't look
> right there. The best you will be able to do on a 3550 is to add some
> monitoring for particular internal DSCP values you want to match on such as
> applying "mls qos monitor dscp 0 8 16 24 32 40 48 56" to the interface you
> are interested in and then "show mls qos int <interfacename> statistics"
>
> Please note that the QoS behaviour between 3550 and 3560 does overlap a bit
> but there are a number of differences you should be aware of since the CCIE
> lab no longer uses the 3550.
>
> Cheers,
> Adam
>
>
>
>
> On Sun, Oct 3, 2010 at 4:17 AM, <dls152_at_cox.net> wrote:
>
> > Hello,
> >
> > I am trying to understand setting layer 2 cos on user interface. I have a
> > machine that is connected to cisco 3550 on access port. I would like to set
> > cos 5 on all the traffic that is coming out on this interface. I read in
> > order to chance cos on packet you need 801.2q tag. I am thinking that the
> > only way to get this tag is from a trunked port? But my user port is access
> > port. Can this be done by just going to the access port and setting the cos
> > on the interface? I did this but i am not sure if this is working because I
> > am reading that the port has to be using 801.2q? Or can I use MQC and match
> > the interface and then set the dscp value for all traffic coming out this
> > interface?
> >
> >
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Received on Sat Oct 02 2010 - 21:30:02 ART