From: Pavel Bykov (slidersv@gmail.com)
Date: Tue Jan 20 2009 - 02:56:53 ARST
When DSCP rewrite is off, that is the "transparancy mode".
Here is excerpt from Cisco to support my claims:
!!!!!!!!!!!!!!! Begin Cisco.com Quote !!!!!!!!!!!!!!!
In Cisco IOS Release 12.2(25)SE or later, the switch supports the DSCP
transparency feature. It affects only the DSCP field of a packet at egress.
By default, DSCP transparency is disabled. The switch modifies the DSCP
field in an incoming packet, and the DSCP field in the outgoing packet is
based on the quality of service (QoS) configuration, including the port
trust setting, policing and marking, and the DSCP-to-DSCP mutation map.
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.
Regardless of the DSCP transparency configuration, the switch modifies the
internal DSCP value of the packet, which the switch uses to generate a class
of service (CoS) value that represents the priority of the traffic. The
switch also uses the internal DSCP value to select an egress queue and
threshold.
!!!!!!!!!!!!!!! End Cisco.com Quote !!!!!!!!!!!!!!!
On Tue, Jan 20, 2009 at 5:25 AM, Raghav Bhargava <raghavbhargava12@gmail.com
> wrote:
> Hi Pavel,
>
> So where does DSCP Transparent mode comes into picture. I agree with your
> steps.
>
> Please shed some more light as it is becoming interesting now..
>
> thanks
> /raghav
>
> On Mon, Jan 19, 2009 at 8:14 PM, Pavel Bykov <slidersv@gmail.com> wrote:
> > 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
> >> >
> >> >
> >> > Blogs and organic groups at http://www.ccie.net
> >> >
> >> >
> _______________________________________________________________________
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> >>
> >>
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> >>
> >> _______________________________________________________________________
> >> Subscription information may be found at:
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> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >
> >
> >
> > --
> > Pavel Bykov
> > ----------------
> > Don't forget to help stopping the braindumps, use of which reduces value
> of
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> >
>
>
>
> --
> Warm Regards
> Raghav
>
-- Pavel Bykov ---------------- Don't forget to help stopping the braindumps, use of which reduces value of your certifications. Sign the petition at http://www.stopbraindumps.com/Blogs and organic groups at http://www.ccie.net
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