class-default traffic drop in congension

[balayanpankaj]

Hi Guys,

Please consider the following example and share your thoughts:

In a MQC deployement, consider that there are following two classes defined:
Class1: EF marking with priority 20%
Class2: AF41 marking with bandwidth 40%

All remaining traffic except above said two classes will go to default class known as class-default.

In case of congestion, which traffic will drop in class-default.

As per my understanding class-default will match all traffic except EF, AF41, CS6 & CS7 (EF is defined in Class1, AF41 is defined in Class2, CS6 & CS7 will be considered as control traffic for 25% reserve bandwidth) and during congestion on all classes (including class-default) class-default will not carry the traffic marked with EF, AF41, CS6 & CS7 instead it will carry the low priority traffic (traffic from class selectors CS0, CS1, CS2, CS3 and CS4 (AF42 + AF43).

Please comment & correct if I am wrong. Do let me know if any other clarity is required on this scenerio.

Thanks.

[SOcchiogrosso]

Are you marking any other traffic besides the traffic that is to be classified in your 2 classes?

I don't know what the goal is but you might want to look at WRED and verify the random detect setting in your configurations.

Sent from Cisco Technical Support iPad App

[Joseph W. Doherty]

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Posting

I believe you'll find ALL traffic but DSCP EF and AF41, for what you describe, will use class-default.

The 25% "reserve" bandwidth you mention may be the pre-HQF CBWFQ default of only allowing 75% explicit bandwidth allocations for class bandwidths (i.e. other than class-default).

[mukeshpal]

Hi Pankaj,

As per your scenario, During congestion if class 2 traffic exceeds 40% bandwidth and then it may move to any other class which is not fully utilized and if defualt class is not congested, it may move to default-class. There could be situation when deafult-class traffic also start dropping at some stage at the peak congestion time. for class 1 max bandwidth is 20 % of available bandwidth if traffic of class 1 is exceeding beyond 20%, class 1 traffic will not move to any other class and it start dropping...

Pls find below link...HTH

http://www.cisco.com/en/US/docs/ios-xml/ios/qos_conmgt/configuration/12-4t/qos-conmgt-oview.html#GUID-A3EE4870-4F4F-4D73-A6FA-765D16055DD3

The sum of all bandwidth allocation on an interface cannot exceed 75 percent of the total available interface bandwidth. The remaining 25 percent is used for other overhead, including Layer 2 overhead, routing traffic, and best-effort traffic. Bandwidth for the CBWFQ class-default class, for instance, is taken from the remaining 25 percent. However, under aggressive circumstances in which you want to configure more than 75 percent of the interface bandwidth to classes, you can override the 75 percent maximum sum allocated to all classes or flows. If you want to override the default 75 percent, exercise caution and ensure that you allow enough remaining bandwidth to support best-effort and control traffic, and Layer 2 overhead.

Regards,

Mukesh

[Joseph W. Doherty]

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The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

mukeshpal wrote:
As per your scenario, During congestion if class 2 traffic exceeds 40% bandwidth and then it may move to any other class which is not fully utilized and if defualt class is not congested, it may move to default-class. There could be situation when deafult-class traffic also start dropping at some stage at the peak congestion time. for class 1 max bandwidth is 20 % of available bandwidth if traffic of class 1 is exceeding beyond 20%, class 1 traffic will not move to any other class and it start dropping...

Perhaps to clarify this, classes may utilize bandwidth not used by other classes, but the traffic will remain in that original class.  In other words, class bandwidth does not set a maximum, it sets a minimum guarantee.

[balayanpankaj]

Thanks Guys for sharing your knowledge.

Yeah CBWFQ can burst into other class and may utilize bandwidth not used by other classes however LLQ cannot burst into other class in case of congestion in LLQ. CBWFQ provides the minimum bandwidth guarantee and LLQ specify the maximum bandwidth available for delay sensetive traffic.

Still my question is unanswered. In simple words, I am looking for clarity on treatment for AF41 marked traffic in class-default during congestion in all classes (class-1, class-2 and class-default). During congestion EF marked traffic will only carried by Class-1 (LLQ). AF41 marked traffic will be carried by Class-2 (CBWFQ) but I am not sure how class-default will treat the AF41 marked traffic during congestion. Will class-default consider AF41 or just drop it? Though my gut feeling says that, class-default will drop the AF41 packet as it is specified under Class-2 and will not be considered by class-default during peak congestion. Will there be any change in this if we configure fair-queue in class-default? By default class-default do the tail droping in congestion instead of weight based droping profiles to change this behaviour we can configure fair-queue under class-default.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Actually LLQ can exceed its bandwidth allocation, if there's no interface congestion.

Class-default does not directly "consider" other class congestion.  Nor do other classes directly "consider" class-default congestion.

Excluding LLQ, what happens when there's congestion, each class is dequeued proportionally to its class (actually queue) weight relative to other classes except for class-default, pre-HQF, with FQ enabled.  The latter, has flow queues which get dequeued, also proportionally, relative to all the other queues.

Each queue, again excluding LLQ, will drop when the number of packets trying to be enqueued exceed the queue depth (in packets) allocated for that queue.

Specifically for your question, class-2, your AF41 marked traffic, will drop if it exceeds it allocated queue size.  By default, this would be tail drop.  If WRED is enabled, WRED will look at the "average" queue depth when tries to enqueue a new packet and determines, based on its settings, whether to drop that packet.

Does class-default, or other classes, have any affect?  Yes, as whatever share of interface bandwidth being otherwise used will not be available to class-2, and when it's not, class-2 may enqueue when it otherwise would not.