RE: Layer 2 QOS COS marking on Inbound

Here's 3 lines I got out of the doc you sent. But I do think you are also correct and it relates to the hardware platform. 3750 thresholds and what queues the traffic goes into for inbound and outbound traffic can be determined by COS or DSCP. Some 6500 blades only use COS and some can do both. Very confusing!!!

1) The switch uses the internal DSCP value to generate a CoS value representing the priority of the traffic.

2) If you configure a port with either of these options and non-IP traffic is received, the switch assigns a CoS value and generates an internal DSCP value from the CoS-to-DSCP map. The switch uses the internal DSCP value to generate a CoS value representing the priority of the traffic.

3) 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

-----Original Message-----
From: Carlos G Mendioroz [mailto:tron_at_huapi.ba.ar]
Sent: Monday, July 18, 2011 10:03 AM
To: Diment, Andrew
Cc: Irfan Sid; Cisco certification
Subject: Re: Layer 2 QOS COS marking on Inbound

Hmm,
I have seen many documents talking about "internal DSCP" and how the
maps go from COS to DSCP (input) to COS again (output).

Actually, you may use COS or DSCP on input (via trust) and not remark
the DSCP.

Take a look at table 31-3 on
http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst2960/software/release/12.2_46_se/configuration/guide/swqos.html

-Carlos

Diment, Andrew @ 18/07/2011 10:52 -0300 dixit:
> Actually you have that a little backwards, switches use COS internally. What that internal COS value is set to depends on what is "trusted" on the interface. If you trust COS (mls qos trust cos) it will use the COS value in the 802.1p field of the incoming packet AND will change the DSCP value using the COS-to-DSCP table. Thatbs why for VoIP traffic you always have to adjust the table with the command "mls qos map cosbdscp 0 8 16 24 32 46 48 56" because the standard says to use DSCP 40 for COS 5 but VoIP uses DSCP 46 and we don't want that overwritten to 40.
>
> If it's the native vlan (no 802.1p tag) or the link is not a trunk the COS value and DSCP value will both be set to zero, which is the default and configurable. You can turn this rewriting off also if you wish. If the interface has "mls qos trust dscp" configured it will use the dscp-to-COS table to determine the internal COS value.
>
> This doc has a very good explanation.
>
> http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml
>
>
> Andy
> CCIE 10685
>
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of Carlos G Mendioroz
> Sent: Monday, July 18, 2011 4:37 AM
> To: Irfan Sid
> Cc: Cisco certification
> Subject: Re: Layer 2 QOS COS marking on Inbound
>
> COS is not carried from input to output, but instead, converted to
> internal DSCP using cos-to-dscp if cos is trusted on input or
> override is used.
>
> On output, its value is mapped using dscp-to-cos and set on the egress
> frame if the frame supports it.
> DSCP is also reset unless pass through is used, or modified by some exit
> policy.
>
> But make yourself a favour and test it. That's the best way to learn it!
>
> -Carlos
>
> Irfan Sid @ 18/07/2011 01:58 -0300 dixit:
>> Hi Group.
>>
>> On the Cisco switches (2960) can I apply layer 2 COS marking inbound on a
>> Dot1q trunk, or can it be applied outbound only?
>>
>> I understand when a Frame comes inbound on a Trunk, the Vlan tag is removed,
>> so dot1p cos cannot be applied.
>>
>> On the outbound, the VLAN tag is added on a trunk and 802.1p COS value can
>> be attached to the frame.
>>
>> Is this understand correct.
>>
>> However, I have seen mls qos cos <value> override, applied on voice vlans
>> on phone ports so not sure if the cos is written to the frame and classified
>> before the vlan tag is removed on the inbound.
>>
>> Please can someone correct my understanding?
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
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>>
>>
>>
>>
>>
>>
>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
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Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
This communication is the property of Qwest and may contain confidential or
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