Re: mls qos map policed-dscp

Hi,

In general, Catalyst switches implement "synchronous" marking logic:
i.e. all potential
QoS labels (L3,L2) are kept synchronized using the DSCP-to-CoS,
CoS-to-DSCP and IP Prec-to-CoS
global mapping tables. This is pretty natural if you think of
"uniform" QoS marking model.
In some cases it is possible to modify just one QoS marking point,
keeping the other intact.

See below for more details.

> Does mls qos map policed-dscp remark DSCP and COS field or just DSCP?

Yes, CoS is modified according to the DSCP-to-CoS mapping table (in
both 3550 and 3560).
You may avoid CoS modification while policing in the 3550 model by
using the configuration
similar to the following:

mls qos cos policy-map
!
policy-map POLICE_INBOUND
 class ICMP
  trust dscp
  set cos 2
  police 64000 16000 exceed-action policed-dscp-transmit

Here you trust DSCP values and set CoS at the same time. Setting CoS
directly in policy-map in only
possible in the 3550 model when enabled by the special global-mode
command. The 3560 does
not provide such flexibility, and most of the time it is not needed.

> if I have interface with just mls qos trust dscp, if port receives packet
> with for example DSCP 46, does it remark also COS field when it sends packet
> to anoter switch over trunk.

Yes, by default CoS is modified according to the DSCP to CoS mapping table.
In the 3550 model you may avoid this behavior by enabling special CoS
pass-thru mode:

interface FastEthernet 0/4
 mls qos trust dscp pass-through cos

(there is symmetric DSCP pass-thru mode, configured using the command
"mls qos trust cos pass-through dscp")

In the 3560 model, it is not possible to disable CoS rewrite. More
than that, you have to set
CoS values for non-IP packets by using the "set dscp" command, as there is
no "set cos" commad. Still, you can disable DSCP rewriting while trusting CoS
(mls qos trust cos) by entering the global command "no mls qos rewrite ip dscp"

HTH,

-- 
Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
petr_at_INE.com
Internetwork Expert, Inc.
http://www.INE.com
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