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3750-X Buffer-Management

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Dec 18th, 2013
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Hi all,


some days ago someone posted a link to a very good document on 3750-X Buffer-management from the Documnets-Area here on CSC. Sadly I didn't bookmark that and now I don't find it any more.


So, please post it again! ;-)

And if anyone has more information on handling microbursts on that platform, I'm also happy to get them.


Thanks in advance.



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Karsten Iwen Thu, 12/19/2013 - 01:19
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Thanks, that was the document I was looking for!



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daniel.dib Thu, 12/19/2013 - 02:32
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If you don't have the need for QoS, running with mls qos disabled performs better than running with mls qos with the default settings.


Properly tuned settings will achieve the best results but finding them can be tricky. I think the new 3650 has double the amount of buffers which is good.


Microburst will often occur if you have 10G uplink and 1G to servers and so on. You can use Portchannels to spread the load which will help with microburst. You could also spread ports on different ASICs since buffers are assigned per ASIC if I remember correctly.


That document is a long but good read. I used it for my CCIE studies.


Pavlo Bykov also created a QoS calculator which you could have a look at:


http://www.bitsonwire.com/?p=51



Daniel Dib
CCIE #37149

Joseph W. Doherty Thu, 12/19/2013 - 03:07
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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.


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Posting


If you don't have the need for QoS, running with mls qos disabled performs better than running with mls qos with the default settings.

Correct, although as Daniel notes, much depends on whether you "need" QoS.


With QoS disabled, all the buffer resources are devoted to all traffic, where with QoS enabled, by default, half the buffers have been "reserved" by the 4 egress class queues leaving less for any one egress queue.  The latter often results in drops not seen with QoS disabled.


Properly tuned settings will achieve the best results but finding them can be tricky.

I would quible with "best".  "Best" in what you can accomplish with the device for QoS, but not "best" is how you might really want to manage congestion.  I do, though, agree with "tricky"!!!  (And not just in understanding the QoS capabilies, but by what can be really accomplished with them.)


That document is a long but good read.

Indeed!  Until I read this document (which I suspect was an internal Cisco document), how common and reserved allocations interacted wasn't clear to me.


PS:

BTW, Cisco not too long ago released a note (on the original 3750 series web page - argh) which notes the 3750-X has additional buffer resources for the uplink ports vs. the other ports.


This is the document: http://www.cisco.com/en/US/products/hw/switches/ps5023/products_qanda_item09186a0080c13273.shtml


And what's of interest is:

The total available common pool for egress buffers  varies from one platform to the other. They are more limited in 2960-S:  2MB for the whole system (downlink ports + uplink ports), while 3750-X  has 2MB for each set of 24 downlink ports and 2MB for uplinks.

daniel.dib Thu, 12/19/2013 - 04:44
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Good points Joseph.


One can only hope that future switches get the QoS of 3650 and 3850 with MQC and the use of queue-limit instead of buffers and so on.


Off topic:


Wish there was a way of liking posts or giving them a rating even if you weren't the author. Many good posts go unrated.



Daniel Dib
CCIE #37149

Karsten Iwen Thu, 12/19/2013 - 05:08
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Time to jump in again ...


the 3750-X in question needs qos, and I think that my qos-settings are fine at the moment. But because QoS is always like black magic I wanted to read much more in-depth material to evaluate the situation. I still have drops, but the next action is to find the source of the bursts.


BTW: you can give a rating to other posts. Thats what the left block of the stars are for.



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Jon Marshall Thu, 12/19/2013 - 05:19
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But because QoS is always like black magic


Slightly off topic I have often wondered how other vendors manage QOS on hardware based platforms. To be honest, QOS can be so complicated on switches it is often better, as noted by Daniel and Joseph,  to just leave it alone because enabling it without a full understanding often leads to worse performance than before.


Cisco seem to be trying to use MQC more on the newer switches and i was wondering whether other vendors, such as Juniper, having had the advantage of starting from scratch in effect, had managed to apply a more consistent approach to QOS on their hardware platforms.


I don't want to hijack the thread but i would be interested to hear others thoughts/experiences with other vendor's switches.


Jon

Karsten Iwen Thu, 12/19/2013 - 05:40
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In practice I don't work with other vendor switches. But I'm for sure happy to see MQC coming to more platforms that do QoS in hardware.



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daniel.dib Thu, 12/19/2013 - 05:44
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Thanks!


I'm still new to this forum so I don't know all of the features yet.


I would be very interested to see something like meet the expert on QoS but focused more on the hardware side, buffer management and things like that, not just the standard stuff on DSCP, CoS and so on. I understand that there are different BUs so there would probably need to be a few Cisco employees involved to cover at least all of the Catalyst platforms.




Daniel Dib
CCIE #37149

Karsten Iwen Thu, 12/19/2013 - 05:52
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There are a couple of presentation on ciscoLive365 about the architecture of the different Catalyst-switches. It's worth looking at these when trying to understand why the switches behave the way they do.



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daniel.dib Thu, 12/19/2013 - 05:57
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Yes, I've seen most of them.


Sometimes not all information is public though, like how much buffers are available per ASICs and so on.


The ME3600/3800 devices has really good QoS as well. Hopefully the Catalysts will from now on have MQC.



Daniel Dib
CCIE #37149

Jon Marshall Thu, 12/19/2013 - 06:01
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Sometimes not all information is public though, like how much buffers are available per ASICs and so on.

And you can almost always guarantee that is the one bit of information you are actually looking for


Jon

Jon Marshall Thu, 12/19/2013 - 06:17
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Karsten

          

There are a couple of presentation on ciscoLive365 about the architecture of the different Catalyst-switches. It's worth looking at these when trying to understand why the switches behave the way they do.

Are these available to anyone or do you need a login for them ?

Jon

Karsten Iwen Thu, 12/19/2013 - 06:35
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You need a login, but i think that it's free for everyone since a couple of years.



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daniel.dib Thu, 12/19/2013 - 06:39
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Just create an account on ciscolive365.com. It's free!


Then do a search for BRKARC or Catalyst 3750 or whatever platform you are interested in. Lots of great stuff there.



Daniel Dib
CCIE #37149

Jon Marshall Thu, 12/19/2013 - 06:47
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Thanks to both of you.


Jon

Joseph W. Doherty Thu, 12/19/2013 - 08:49
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i was wondering whether other vendors, such as Juniper, having had the advantage of starting from scratch in effect, had managed to apply a more consistent approach to QOS on their hardware platforms.

Ask me that in about 6 months - we're going to be doing massive deployment of brand J along side our brand C.


PS:

One great feature of brand J, all their (different model) devices are configured exactly alike, except when they are not. 

Jon Marshall Thu, 12/19/2013 - 05:04
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Daniel


Wish there was a way of liking posts or giving them a rating even if you weren't the author. Many good posts go unrated.

Not sure what you mean by this. If you are not the author of the post you cannot mark the answer as correct but you are free to rate any post whether or not you are the author or indeed even involved in the thread.  Just hover over the stars for the question you want to rate and choose the rating. You can also rate a thread that has already been rated, that is why you might have seen a particular post get more than one rating.


In fact, many if us do rate posts if we see a good answer.


The only thing you cannot do is rate a question and there are times when the question itself deserves a rating.


Jon

Joseph W. Doherty Thu, 12/19/2013 - 08:38
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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


One can only hope that future switches get the QoS of 3650 and 3850 with MQC and the use of queue-limit instead of buffers and so on.

Although MQC makes for a more consistency, is some ways, it's lipstick on a pig.  L3 switches are very much limited by the capabilities of the hardware.  You'll often find many MQC commands work on an ISR but not on a L3 switch that does MQC.


Very recently, we're starting to deploy sup2Ts, and their MQC supports also depends on line card.  So, we're staying with WRR type commands, which seem to work, as expected, with both the older and newer line cards. 

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