[c-nsp] internal DSCP

P.A razor at meganet.net
Thu Aug 12 15:40:55 EDT 2010

Tim thanks for your response, I think I'm starting to get it.

So basically if you do ingress marking with an ACL by default it will not
use that marking on egress unless you use an egress acl for remarked packets
using 'platform ip features sequential'?

What about for the following, would the set dscp value be used?

class-map match-any voip_traffic
  match protocol rtp audio

Policy Map incoming_voip_policy
  Class voip_traffic
    trust dscp
   police cir percent 20 conform-action set-dscp-transmit ef ....

thanks, Paul

-----Original Message-----
From: Tim Stevenson [mailto:tstevens at cisco.com] 
Sent: Thursday, August 12, 2010 2:34 PM
To: P.A; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] internal DSCP

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:

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,

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.
>im confused to which is true, see below.
>"Upon output, the internal DSCP settings are copied to the IP header. If
>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)
>was present on the arriving packet to be used for classification purposes."
>cisco-nsp mailing list  cisco-nsp at puck.nether.net
>archive at 

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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