Re: Layer 2 qos

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 Sun Oct 03 2010 - 10:29:55 ART