[c-nsp] internal DSCP

[Tim Stevenson]

Hi PA,

Not sure where your quotes are coming from. The 2nd one is a bit 
misleading IMO, it sort of mangles the concepts of PFC classification 
& egress port QoS.

It refers to a specific behavior around matching remarked DSCP in an 
egress ACL. That is not possible w/o a recirculation.

This section of the cfg guide should help:
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/qos.html#wp1766015

The point is, if you remark DSCP on ingress, an egress ACL will not 
match on the remarked DSCP without first recirculating the packet.

Regardless of that, the remarked DSCP (or derived COS) will 
definitely be used to map the packet to an egress port queue and that 
remarked DSCP will be carried in the final packet.

Hope that helps,
Tim



At 08:41 AM 8/12/2010, P.A averred:



>I have a questing about internal DSCP on a 6500 that I'm not really sure
>about. I know that it's used to identify the priority of a  frame/packet as
>it transits the switch but I have read on some sites that the internal DSCP
>is copied to the frame/packet as it leaves the switch. On other sites that
>when the packet arrives at the egress port the original ToS will be used. So
>im confused to which is true, see below.
>
>
>
>"Upon output, the internal DSCP settings are copied to the IP header. If the
>outbound frame is being given a trunking header (ISL or 802.1Q) then the
>appropriate CoS bits are also set."
>
>
>
>"7.12 Egress ACL Support for Remarked DSCP
>
>When a frame transits the switch, classification performed by the PFC can
>change the ToS priority setting (IP Precedence or DSCP) in the packet. When
>the packet arrives at the egress port however, the default action will
>result in the original ToS priority (not the PFC remarked ToS priority) that
>was present on the arriving packet to be used for classification purposes."
>
>
>
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Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Distinguished Technical Marketing Engineer, Cisco Nexus 7000
Cisco - http://www.cisco.com
IP Phone: 408-526-6759
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