RE: Layer 2 QOS COS marking on Inbound

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?
>
>
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--
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net