From: Pavel Bykov (slidersv@gmail.com)
Date: Tue Jan 20 2009 - 02:14:38 ARST
Darby:
*Cat3560(config-if)# mls qos trust [cos | dscp ] does the same thing per
interface*
No, I don't think it does. This command sets the trust mode. Here is my
understanding of it with quick example with trust CoS and DSCP rewrite
(without complicating thing too much).
1.
Trust setting: Trust CoS
DSCP rewrite setting: OFF
Incoming Packet: Cos = 0, DSCP = 46
Packet comes in, CoS > DSCP map is 0 > 0. Internally, CoS is 0, DSCP is also
0. Queue is selected based on that. Packet is placed into a Queue with CoS
0, but since rewrite was off, DSCP was left the same at 46.
Output Packet: CoS = 0, DSCP = 46
2.
Trust setting: Trust CoS
DSCP rewrite setting: ON
Incoming Packet: Cos = 0, DSCP = 46
Packet comes in, CoS > DSCP map is 0 > 0. Internally, CoS is 0, DSCP is also
0. Queue is selected based on that. Packet is placed into a Queue with CoS
0.
Since rewrite is on, DSCP is set to calculated value, i.e. 0
Output Packet: CoS = 0, DSCP = 0
3.
Trust setting: Trust DSCP
DSCP rewrite setting: OFF
Incoming Packet: Cos = 0, DSCP = 40
Packet comes in, DSCP > Internal DSCP map is used, where 40 maps to 46.
Queue is selected based on that. Packet is placed into a Queue with CoS,
that is calculated from DSCP > CoS map, where 46 maps to 5. Notice, 46 is
used for calculation.
But since rewrite is off, 40 is left in the header. DSCP is not set to
calculated value.
Output Packet: CoS = 0, DSCP = 40
4.
Trust setting: NO TRUST (default)
DSCP rewrite setting: OFF
Incoming Packet: Cos = 0, DSCP = 46
Packet comes in, CoS > DSCP map is 0 > 0. Internally, CoS is 0, DSCP is also
0. Queue is selected based on that. Packet is placed into a Queue with CoS
0.
Since rewrite was off, DSCP was left the same at 46.
Output Packet: CoS = 0, DSCP = 46
On Tue, Jan 20, 2009 at 12:33 AM, Darby Weaver <ccie.weaver@gmail.com>wrote:
> *DSCP Transparency *
>
> *DSCP Transparency is a Cat3560 feature.*
>
> Global command that is disabled by default. If DSCP transparency is enabled
> by using the no mls qos rewrite ip dscp command, the switch does not modify
> the DSCP field in the incoming packet, and the DSCP field in the outgoing
> packet is the same as that in the incoming packet.
>
> Cat3560(config)# mls qos rewrite ip dscp
>
> disable DSCP transparency to allow switch to modify DSCP values based on
> trust or ACL
>
> -OR-
>
> Cat3560(config)# no mls qos rewrite ip dscp
>
> Cat3560(config-if)# mls qos trust [cos | dscp ] does the same thing per
> interface
>
> *Passthrough Option is a 3550 Feature:*
>
> Cat3550(config-if)# mls qos trust [cos | dscp] pass-through [ dscp | cos]
>
> Forces Cat to treat CoS and DSCP independently. So, it trusts one and
> doesn't change the other marked as pass-through.
>
>
>
> On 1/19/09, Raghav Bhargava <raghavbhargava12@gmail.com> wrote:
> >
> > HI experts,
> >
> > I am not able to understand to the DSCP Transparency Mode in QOS. Can
> > anyone shed some light on it.
> > I am studying it from the Configuration Guide of Catalyst 3560.
> >
> > --
> > Warm Regards
> > Raghav
> >
> >
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>
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