Access switch buffers

[tonypearce1]

Could someone shed some light on how switch buffers are utilised for access switches like the 3750?

I can find plenty of information regarding router buffers which is good but how does this process translate to pure layer 2 switching? For example multiple innput interfaces with a single output interface, which is oversubscribed.

Also, how are buffers allocated? If for example a 1500 byte packet is queued for any reason, does this packet use the same or less buffers than a 9000 byte packet? I.e does packet size matter with regards to buffer space?

I understand figures on buffer allocation on the 3750 are not published, but are buffers tunable? If so how can I find command references on this? I have used the "hold-queue" command on routers. This command takes effect on the 3750 interface however I never see that queue figure increase so do not think this is the correct thing to do or correct way of achieving what I am after

I've tried to find information on this using the Cisco search tool and Google to no avail so hoping someone can point me in the direction of some reading material at least I would really appreciate that.

Thanks

[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.

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Posting

There are a couple of white papers on Cisco's web site that do explain buffer usage for the 3750; although perhaps, I thought, a bit unclear.

Recently, there was an excellent posting somewhere in these forums that further explained 3750 buffer usage.

If you're unable to find these, let me know, and I'll see if I can find their links (assuming someone else doesn't post their links).

PS:

Forgot to mention, they are tunable.

[tonypearce1]

I've done some more digging and I can find that the buffers are tunable but you cannot change their size per port. The buffers are fixed, and with qos enabled you can tune the buffers per queue as a percentage of the fixed buffer. Without qos enabled there is one FIFO queue. Is this correct?

Lastly, does packet size have any effect on the number of buffers used? For example if a 1500 byte packet arives and is queued, does the switch use "one buffer"? How about a smaller packet say 150 byte? And what about a 9000 byte packet? Does that still only use one buffer or does it use more to hold that one packet? Plus, when the MTU is changed on the switch (and reloaded!) does this affect the shared buffer? Meaning, does this change give you less buffers? The reason I think this is because the shared memory is fixed but this might be explained in itself if I knew how buffers are utilised i.e. one buffer is one packet OR one packet could be any number of buffers depending on its size?

Thanks!

[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

I believe that's correct, buffers are not directly tunable per port, but are tunable per queue-set and then you can assign the port to a queue set.  Unfortunately, there's only two queue sets, likely thinking was one set for access ports and one set for uplink ports.

I also believe that correct, i.e. one FIFO queue if QoS not enabled.

Don't recall reading about how packets of various sizes are mapped into the buffer space.  However, did find the posting I had in mind, see: https://supportforums.cisco.com/docs/DOC-8093  Also found a Cisco doc I had in mind, see: http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml

[tonypearce1]

Thank you for this information and the links. I have some reading to do.