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New Member

3750g-12s output drops

Hello,

We are experiencing output drops on our 3750G-12S (IOS 12.2.55) and I could need some help troubleshooting.

I have monitored throughput with MRTG, but it doesn't seem that high compared to that the 3750G is supposed to handle 17.8 Mpps.

The 3750G's are stacked, running ospf, qos, multicast (IPTV) and etherchannel.

Question: Is the maximum throughput reduced by stacking or any of the other features?

I don't know if one can estimate what the maximum throughput is per ASIC or per port, but on one of the ASICs that is having drops the peak unicast load is 1.3 Mpps and multicast Mpps is 0.9 Mpps. Total ~2.2 Mpps.

This doesn't seem high at all and I don't understand why the drops are appearing.

Question: Does anyone know what I can do to find out the reason for these drops?

5 REPLIES
Super Bronze

Re: 3750g-12s output drops

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

Could be transient microbursts.  These won't normally show in "routing" monitoring.

Since you mentioned you're doing QoS, QoS divides up its buffer resources to service 4 queues per port instead of just 1.  This can lead to insufficient buffer space for some queues.  This might be mitigated with some manual buffer tuning.

You also mention using Etherchannel, but are you getting optimal link balancing?

As to limitations on the original 3750 series.  You can run into performance issues if you exceed hardware resources, i.e. TCAM.  This is best avoided using the correct SDM template.  (NB: -12S offers a template unique to it.)

Another performance limitation is StackWise.  StackWise puts all taffic on the ring and it's the source that takes it back off the ring, not the destination.  (NB: Stackwise+ on the E/X series is "smarter".)  Stacks with more and more members can overload the stack ring.  Stackwise is spec'ed as 32 Gbps, but its actually dual 8 Gbps, duplex.

New Member

Re: 3750g-12s output drops

I will try to adjust the buffers so that queue 4 (which is where the drops are happening) gets more to work with.

I don't know if there are any recommended ways to ajust the buffers, so I'll just try to increase it for queue 4 and observe what happens...

Queueset: 2
Queue     :       1       2       3       4
----------------------------------------------
buffers   :      20      20      20      40
threshold1:     100     200     100     150
threshold2:     100     200     100     150
reserved  :      50      50      50     100
maximum   :     400     400     400     600

TCAM/default SDM template looks OK, I don't think we are using up all the resources. And the default template seems like it's the most fitting.

Super Bronze

Re: 3750g-12s output drops

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

This might help: https://supportforums.cisco.com/docs/DOC-8093

New Member

Re: 3750g-12s output drops

Thanks!

Adjusting the output buffers removed the drops.

I'm glad we solved it, but can't help to wonder why the default setting for buffer is set that low.

Super Bronze

Re: 3750g-12s output drops

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

QoS settings often need to be "tuned" for particular instances.  One size often does not fit all.

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