qos

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Feb 24th, 2010
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i had a question regarding QOS in layer2 mode. i have about 3 3560's in between 2 routers and i'm marking traffic at these edge routers with dscp. if I trust dscp in between both routers and all three switches will the switches automatically do the dscp-to-cos mapping or will i have to trust cos between the switches? Is mls qos rewrite ip dscp global command same as mls gos trust dscp on an individual interface? on the 3560 which when i try to create a class-map to match traffic i get ip dscp as one option, will the switch really match traffic against dscp in layer2 mode?


thanks.

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Jon Marshall Wed, 02/24/2010 - 14:43
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flyinhorse wrote:


i had a question regarding QOS in layer2 mode. i have about 3 3560's in between 2 routers and i'm marking traffic at these edge routers with dscp. if I trust dscp in between both routers and all three switches will the switches automatically do the dscp-to-cos mapping or will i have to trust cos between the switches? Is mls qos rewrite ip dscp global command same as mls gos trust dscp on an individual interface? on the 3560 which when i try to create a class-map to match traffic i get ip dscp as one option, will the switch really match traffic against dscp in layer2 mode?


thanks.


 Is mls qos rewrite ip dscp global command same as mls gos trust dscp on an individual interface? 


No it isn't. If you trust DSCP on an indivdual interface the DSCP could still be changed depending on what the QOS maps say. With "no mls qos rewrite" the switch will leave the DSCP value untouched no matter what the maps say.


If you trust DSCP on the ingress then the switch will still have to use the DSCP-to-CoS map to queue the packet outbound but that doesn't mean you then need to trust cos on ingress. You can trust DSCP on all ingress ports on the 3560s.


Yes a 3560 can use the DSCP value even at L2.


Jon

flyinhorse Thu, 02/25/2010 - 12:06
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so this is what i'm seeing when i mark traffic on my router and trust dscp on the switch. The packets come in as dscp ef and leave as dscp 1 or 2. below is the config and some qos results on the switch. Basically i just issued the auto qos command and wanted the switch to read the dscp vlaue and put it in the appropriate queue. i did not want the switch to remark. but it seems like its doing it. i even have ip dscp rewrite disbaled.



interface FastEthernet0/1

description Cisco ASNGW

switchport trunk encapsulation dot1q

switchport mode trunk

speed 100

duplex full

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape  10  0  0  0

priority-queue out

mls qos trust dscp

auto qos voip trust

spanning-tree portfast trunk



va_ashburn_BaseStation_sw#show mls qos inter fa0/1 statistics

FastEthernet0/1


  dscp: incoming 

-------------------------------


  0 -  4 :          72            0            0            0            0 

  5 -  9 :           0         3155            0            0            0 

10 - 14 :           0            0            0            0            0 

15 - 19 :           0            0            0            0            0 

20 - 24 :           0            0            0            0          127 

25 - 29 :           0            0            0            0            0 

30 - 34 :           0            0            0            0            0 

35 - 39 :           0            0            0            0            0 

40 - 44 :           0            0            0            0            0 

45 - 49 :           0         7175            0            0            0 

50 - 54 :           0            0            0            0            0 

55 - 59 :           0            0            0            0            0 

60 - 64 :           0            0            0            0 

  dscp: outgoing

-------------------------------


  0 -  4 :       30479         2750            0            0            0 

  5 -  9 :           0            9            0            0            0 

10 - 14 :           0            0            0            0            0 

15 - 19 :           0            0            0            0            0 

20 - 24 :           0            0            0            0            0 

25 - 29 :           0            0            0            0            0 

30 - 34 :           0            0            0            0            0 

35 - 39 :           0            0            0            0            0 

40 - 44 :           0            0            0            0            0 

45 - 49 :           0            0            0            3            0 

50 - 54 :           0            0            0            0            0 

55 - 59 :           0            0            0            0            0 

60 - 64 :           0            0            0            0 

  cos: incoming 

-------------------------------


  0 -  4 :       11306            0            0            0            0 

  5 -  7 :           0            0            0 

  cos: outgoing

-------------------------------


  0 -  4 :       43084            0            0            0            0 

  5 -  7 :           7            0            0 

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Jon Marshall Thu, 02/25/2010 - 12:28
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Basically i just issued the auto qos command and wanted the switch to read the dscp vlaue and put it in the appropriate queue.


3560 switches  as with most switches do not queue based on the DSCP value directly. They use the CoS value or derived CoS value to queue the packets.


Couple of things -


1) can you post version of software you are using because behaviour changed halfway through release


2) can you post the QOS map info ie.


sh mls qos maps cos-dscp

sh mls qos maps dscp-cos


Jon

flyinhorse Thu, 02/25/2010 - 12:32
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Cisco IOS Software, C3560 Software (C3560-IPBASE-M), Version 12.2(35)SE5, RELEASE SOFTWARE (fc1)



va_ashburn_BaseStation_sw#show mls qos map cos-dscp

   Cos-dscp map:

        cos:   0  1  2  3  4  5  6  7

     --------------------------------

       dscp:   0  8 16 26 32 46 48 56



va_ashburn_BaseStation_sw#show mls qos map dscp-cos

   Dscp-cos map:

     d1 :  d2 0  1  2  3  4  5  6  7  8  9

     ---------------------------------------

      0 :    00 00 00 00 00 00 00 00 01 01

      1 :    01 01 01 01 01 01 02 02 02 02

      2 :    02 02 02 02 03 03 03 03 03 03

      3 :    03 03 04 04 04 04 04 04 04 04

      4 :    05 05 05 05 05 05 05 05 06 06

      5 :    06 06 06 06 06 06 07 07 07 07

      6 :    07 07 07 07

Nicholas Poole Wed, 05/19/2010 - 03:28
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the C3560/3750 and similar switches can trust CoS or DSCP, so if trust is used they will queue based on this trusted QoS label.  Unless you have a dot1q trunk running you wont have any CoS values to trust anyway.  So if you are simply trusting DSCP, then the switches do not change it and queue on egress via DSCP.  So if all switches in a row are the same config your QoS markings should be intact end to end.  DSCP mutation will not come into effect as you dont want to change DSCP values, so there is no need to have this.


If you do "sh mls qos interface x/y statistics" for each inbound and outbound interface on all 3 switches you should roughly see the same traffic in and out between all switches.  (Obviously if you have QoS marked traffic traversing this from other sources in the switches it wont be just the router DSCP traffic you see)

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