From: Darby Weaver (ccie.weaver@gmail.com)
Date: Tue Jan 20 2009 - 04:26:11 ARST
In Effect, DSCP Transparency is the process by which a packet does not get
re-marked by simply traversing through a switch or other device.
As a feature this command helps to ensure that a packet is not being
re-written such as in the case where we might employ a DSCP Permutation Map.
On 1/19/09, Pavel Bykov <slidersv@gmail.com> wrote:
>
> When DSCP rewrite is off, that is the "transparancy mode".
>
>
> http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_46_se/configuration/guide/swqos.html#wp1331378
>
> 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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>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >
>> >
>> >
>> > --
>> > 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/
>> >
>>
>>
>>
>>
>> --
>> 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/
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