srr-queue bandwidth share and shape questions

Unanswered Question
Oct 19th, 2009

Hi,

I would like some clarification on the srr-queue bandwidth share and shape commands.

I enabled auto qos voip on an interface while doing some testing and the following config was generated;

interface GigabitEthernet0/1

switchport access vlan 13

switchport mode access

switchport voice vlan 25

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

queue-set 2

mls qos trust device cisco-phone

mls qos trust cos

auto qos voip cisco-phone

spanning-tree portfast

So my understanding of the srr-queue bandwidth share 10 10 60 20 command is that; there are four queues per inteface, and the weight values are the frequency in which packets are removed from their queue and sent. Is that an accurate description?

So if the above is correct, a heigher weight would mean packets are dequeued at a higher rate, meaning that queue has higher priority, correct?

Now another question - there are four queues configured, how do you know which queue VoIP traffic will use? And what traffic gets sent over the other three queues?

If the srr-queue bandwidth shape 10 0 0 0 command is applied, it would seem that the VoIP traffic would use queue 1, since you would probably want to shape VoIP traffic to keep it smooth, and shaping is applied on queue 1 in the config I posted above. And i believe that if shapping and sharing is configured shaping is used, not sharing.

So am I on the right track with my assumptions above?

thanks

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Edison Ortiz Mon, 10/19/2009 - 14:37

So my understanding of the srr-queue bandwidth share 10 10 60 20 command is that; there are four queues per inteface, and the weight values are the frequency in which packets are removed from their queue and sent. Is that an accurate description?

4 egress queues, that's correct.

So if the above is correct, a heigher weight would mean packets are dequeued at a higher rate, meaning that queue has higher priority, correct?

yes, on every cycle - 10 packets are serviced from Queue 1, 10 packets are serviced from Queue 2, 60 packets are serviced from Queue 3 and 20 packets are serviced from Queue 4.

Now another question - there are four queues configured, how do you know which queue VoIP traffic will use? And what traffic gets sent over the other three queues?

Queue 1, please refer to the documentation:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/configuration/guide/swqos.html#wp1202482

Table 36-10

If the srr-queue bandwidth shape 10 0 0 0 command is applied, it would seem that the VoIP traffic would use queue 1, since you would probably want to shape VoIP traffic to keep it smooth, and shaping is applied on queue 1 in the config I posted above. And i believe that if shapping and sharing is configured shaping is used, not sharing.

Correct. With Shaping enabled, Queue1 is removed from the 'sharing' and any packet entering Queue1 will be de-queued first but shaped at 1/10 of the interface bandwidth.

You also need to enable Priority Queueing out which is missing from the config.

The 3 remaining queues, will continue sharing the remaining bandwidth as you have 0 as the weight on the shaping command.

Regards

Edison.

LucaSalvatore_2 Mon, 10/19/2009 - 14:43

Ah so acording to the link you posted the queue that gets used depends on whatever CoS value is specified.....

The config I posted was generated by the auto qos voip commands, shouldn't it know to put the 'priority queueing out' command on the interface?

Edison Ortiz Mon, 10/19/2009 - 15:30

Ah so acording to the link you posted the queue that gets used depends on whatever CoS value is specified.....

Correct.

The config I posted was generated by the auto qos voip commands, shouldn't it know to put the 'priority queueing out' command on the interface?

That issue was corrected with 12.2(40)SE or later.

It is mentioned in the link I posted

"Beginning with Cisco IOS Release 12.2(40)SE, Auto-Qos VoIP uses the priority-queue interface configuration command for an egress interface. You can also configure a policy-map and trust device on the same interface for Cisco IP phones."

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/configuration/guide/swqos.html#wp1689696

Regards

Edison

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LucaSalvatore_2 Thu, 10/22/2009 - 12:58

Hi again,

Just regarding the priority-queue out command.

Does this mean that queue 1 will be the only queue serviced while there is data in there? Or does it just mean as soon as a packet arrives on queue 1 it will be serviced no matter what else is happening?

LucaSalvatore_2 Thu, 10/22/2009 - 19:05

ok thanks.

Another question -

the commands;

mls qos srr-queue output cos-map queue 1 threshold 3 5

mls qos srr-queue output cos-map queue 2 threshold 3 3 6 7

mls qos srr-queue output cos-map queue 3 threshold 3 2 4

mls qos srr-queue output cos-map queue 4 threshold 2 1

mls qos srr-queue output cos-map queue 4 threshold 3 0

which were all generated by the auto qos voip config, what thoes the threshold keyword refer to? Is it the PHB?

And also, we know that anything with CoS 5 is going to go to queue 1, so we have to make sure the the VoIP phones mark their VoIP packets with CoS 5.

However, there is nothing in the config to classify other data with CoS values. So even though auto qos voip is configured do you still need to configure class-maps and policy-maps to mark other traffic with the appropriate CoS values, so they get put in the correct queue?

Thanks again

Edison Ortiz Fri, 10/23/2009 - 05:25

what thoes the threshold keyword refer to?

Picture 3 buckets of water placed next to each other. One small bucket, One medium bucket, One large bucket. All buckets belong to the same queue.

If you want to prevent flows with certain QoS markings to drop in the queue, you will place those flows in the large bucket - right?

For less important flows, you would place them on the small or medium bucket right?

Well, that's how you treat the threshold values. Once the bucket is full, it will drop the flows. The smaller the bucket, the sooner flows will start to drop.

On the 3xxx switches, there are 3 thresholds per queues and 2 thresholds are configurable (threshold 1 and 2). Threshold 3 is set to 100% and can't be modified. That's your large bucket.

However, there is nothing in the config to classify other data with CoS values. So even though auto qos voip is configured do you still need to configure class-maps and policy-maps to mark other traffic with the appropriate CoS values, so they get put in the correct queue?

There are default values that works under most implementations, see:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/configuration/guide/swqos.html#wp1202482

Table 36-10

Regards

Edison.

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