QoS on the 6500

Answered Question
Nov 10th, 2008
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Hi, I hope someone can help, this is a straight forward question, I'm marking my traffic using DSCP values for my 6500 and 4500 switches.

Does marking critical traffic with high dscp values ensure that this traffic will be automatically prioritized into high priority queues? Thanks to all

Correct Answer by allan.thomas about 8 years 5 months ago

The cos-dscp map is a one to one mapping and ensures that the appropriate DSCP value is written in the IP Packet when trusting CoS.


In this example mls qos map cos-dscp 0 10 18 24 34 46 48 56, CoS 3 is mapped to DSCP 24 and CoS 5 is mapped to 46.


However, if you do decide to trust DSCP, then the CoS value is derived from the default dscp-cos map for egress scheduling as below:-


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


As I have mentioned previously, when you enable mls qos, CoS values are automatically mapped to individual queues, this is completely independant of what your DSCP marked value is.


Essentially you define which CoS values are mapped to your queues, and depending on how your traffic is classified will determine which queue it placed in.


For example, a 6748 module has the transmit queue notation 1p3q8t, this means that each port has a single priority queue and three queues each with 8 thresholds.


The show queueing interface x/y command I mentioned previously indicates which queue and threshold these CoS values have been mapped. As in the example below:-


Q T Cos

1 1 0

1 2 1

1 3

1 4

1 5

1 6

1 7

1 8

2 1 2

2 2 3 4

2 3

2 4

2 5

2 6

2 7

2 8

3 1 6 7

3 2

3 3

3 4

3 5

3 6

3 7

3 8

4 1 5


In this example, Q1 is Low priority, Q2 is Medium, Q3 is High and Q4 is the strict priority-queue.


You can see that CoS 3 is mapped to Queue 2 threshold 2 by default, however say you require CoS 3 to have higher priority over CoS 4, then you will need to configure wrr-queue map to move CoS 3 to either Q3 or higher threshold in Q2.


In your case if you trust DSCP traffic and you have classified a certain application with DSCP24, then the dscp-cos map will mark the CoS value as 3. In this instant all your DSCP24 traffic will fall into Q2 T2.


Hope this helps.

Allan.


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shevchiksv Mon, 11/10/2008 - 12:36
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not at all.

You must explicitly insert traffic, for example, with voice ef to individual queue

What congestion management mechanism do you apply?

allan.thomas Mon, 11/10/2008 - 14:36
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Although congestion can typically occur on egress, it can also occur on ingress.


Therefore it would be recommended to take advantage of the ingress queuing capabilities of the hardware by trusting CoS.


Trusting DSCP will forward the packet to the PFC and not use ingress scheduling, whereas CoS will. Simply ensure that you have configured your appropriate cos-dscp mapping.


When mls qos is enabled, CoS values are mapped to either a different queue, or the same queue but different threshold. In most case more that one CoS value will be a assigned to queue as there are typically less queues than CoS values.


To verify the default mapping, run the command 'show queueing interface x/y'. This will show which CoS values are mapped to which queue and threshold.


By default EF traffic will be mapped to the strict priority-queue. If the CoS mappings are not appropriate then you can re-assign them to the appropriate queue/threshold with the 'wrr-queue cos-map' command.


Hope this helps.

Allan.


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Borman Bravo Tue, 11/11/2008 - 06:22
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Allan,


So adding the cos-dscp mapping will ensure that all dscp marked traffic is assigned to an specific queue because it is assigned by default when COS marking is used? thanks.

Correct Answer
allan.thomas Tue, 11/11/2008 - 07:20
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The cos-dscp map is a one to one mapping and ensures that the appropriate DSCP value is written in the IP Packet when trusting CoS.


In this example mls qos map cos-dscp 0 10 18 24 34 46 48 56, CoS 3 is mapped to DSCP 24 and CoS 5 is mapped to 46.


However, if you do decide to trust DSCP, then the CoS value is derived from the default dscp-cos map for egress scheduling as below:-


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


As I have mentioned previously, when you enable mls qos, CoS values are automatically mapped to individual queues, this is completely independant of what your DSCP marked value is.


Essentially you define which CoS values are mapped to your queues, and depending on how your traffic is classified will determine which queue it placed in.


For example, a 6748 module has the transmit queue notation 1p3q8t, this means that each port has a single priority queue and three queues each with 8 thresholds.


The show queueing interface x/y command I mentioned previously indicates which queue and threshold these CoS values have been mapped. As in the example below:-


Q T Cos

1 1 0

1 2 1

1 3

1 4

1 5

1 6

1 7

1 8

2 1 2

2 2 3 4

2 3

2 4

2 5

2 6

2 7

2 8

3 1 6 7

3 2

3 3

3 4

3 5

3 6

3 7

3 8

4 1 5


In this example, Q1 is Low priority, Q2 is Medium, Q3 is High and Q4 is the strict priority-queue.


You can see that CoS 3 is mapped to Queue 2 threshold 2 by default, however say you require CoS 3 to have higher priority over CoS 4, then you will need to configure wrr-queue map to move CoS 3 to either Q3 or higher threshold in Q2.


In your case if you trust DSCP traffic and you have classified a certain application with DSCP24, then the dscp-cos map will mark the CoS value as 3. In this instant all your DSCP24 traffic will fall into Q2 T2.


Hope this helps.

Allan.


Pls rate helpful posts.

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