[c-nsp] incoming queue

[Tim Stevenson]

If you are using COS-Q mapping, the frame *must* 
carry COS to be prioritized on ingress. If using 
DSCP-Q (available on certain 10G modules) then we 
can prioritize on the ingress port based on DSCP, 
which of course would always be carried in an IP packet.

An input service-policy is applied in the PFC/DFC 
- ie, 'long after' the packet was ingress queued. 
Clearly ingress/egress service policies, trust 
statements, etc affect how egress queueing is 
handled and what's carried in the final packet.

In the vast majority of cases, input queuing just 
isn't that useful as queueing only happens when 
there's congestion - and congestion is more 
likely by far on egress where you can have n-to-1 traffic patterns.

Tim

At 12:39 PM 8/20/2010, P.A contended:

>Yeah I have the same config, but don’t think 
>that incoming packets will be mapped to the 
>priority queue. I think all the config below is 
>going to do is set the internal dscp with the 
>policy to be used on the outgoing queue. From 
>what I understand from the link below all 
>incoming packets 1st are assigned to an incoming 
>queue before anything else. So unless the packet 
>already has a COS marking it will not use the priority queue.
>
>-----Original Message-----
>From: Peter Rathlev [<mailto:peter at rathlev.dk>mailto:peter at rathlev.dk]
>Sent: Friday, August 20, 2010 2:47 PM
>To: P.A
>Cc: cisco-nsp
>Subject: Re: [c-nsp] incoming queue
>
>Hi Paul,
>
>(I've Cc'ed the list again since other people's eyes can spot errors I
>might have made.)
>
>On Fri, 2010-08-20 at 12:51 -0400, P.A wrote:
> > Actually Peter, I have come to determine that even now with the
> > correct cos 5 mapped to the incoming priority queue, unless it’s a
> > trunk it will not pass any packets to that queue.
> >
> > See,
> > 
> <http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html#wp1740736>http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html#wp1740736
>
>A service policy is applied outside of this diagram (afterwards). Take a
>look at figure 41-6 on the same page (anchor wp1744018 on same URL). As
>far as I know a service-policy will always override port trust (and
>CoS/DSCP mappings).
>
>And I see there's even a very simple way to trust DSCP via a policy-map.
>I tested this configuration successfully:
>
>! *** Router ***
>policy-map Trust-DSCP-pmap
>  class class-default
>   trust dscp
>   exit
>  !
>  exit
>!
>interface GigabitEthernet1/1
>  description Towards test host
>  switchport
>  switchport access vlan 3
>  switchport mode access
>  mtu 9216
>  load-interval 30
>  mls qos trust cos
>  spanning-tree portfast edge
>  service-policy input Trust-DSCP-pmap
>  exit
>!
>
>And it works fine, everything comes through with correct DSCP/ToS.
>Removing the service-policy makes everything CoS 0 / DSCP 0; trust DSCP
>always lets things through. (Verified with "ping -Q <tos> ..." and a
>tcpdump on the other end.)
>
>Thus you can "trust DSCP" on an access port and still be able to
>configure a priority-queue on input (i.e. have "mls qos trus cos" on the
>interface).
>
>This test was on a PFC3B, module is a WS-X6748-GE-TX without DFC and the
>system is running SXI/AIS. The WS-X6748-GE-TX doesn't have a
>priority-queue on input (it's 1q8t) so I couldn't test that part, but I
>can see no reason why it wouldn't work exactly the same way on other
>modules and other software. I tested it on a WS-X6516-GBIC (1p1q4t) and
>SXF/AES, but I don't have a host on that box to verify with. It didn't
>complain about the configuration though.)
>
>
>P.S.: If you want to trust only some DSCP values (or for some other
>reason want to keep the class-map model) you can bundle DSCP values:
>
>class-map match-any Trust-DSCP-cmap
>  match ip dscp 0 1 2 3 4 5 6 7
>  match ip dscp 8 9 10 11 12 13 14 15
>  match ip dscp 46 63
>  exit
>!
>policy-map Trust-DSCP-pmap
>  class Trust-DSCP-cmap
>   trust dscp
>   exit
>  !
>  class class-default
>   set ip dscp 0
>   exit
>  !
>!
>interface GigabitEthernet1/1
>  [...]
>  service-policy input Trust-DSCP-pmap
>  exit
>!
>
>This trusts only DSCP 0-15, 46 and 63.
>
>--
>Peter
>
>
>
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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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