[c-nsp] Constant output drops on etherchannel

[Tóth András]

Cisco Catalyst 3750 QoS Configuration Examples is also a decent
documentation, can be found on the following link.
http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml


On Mon, Jan 17, 2011 at 12:27 AM, Dan Letkeman <danletkeman at gmail.com> wrote:
> Nick,
>
> Thanks for the detailed explanation.
>
> The problem is I also see this on our gig switches as well.  And only
> on ether channel's, not on a single interconnects.  The traffic can be
> a such a minimum and I still see drops.
>
> I would like to tune the output buffers, but I'm not sure where to
> start.  I know that I need to learn some more about qos, because we do
> have a voice network that is growing very fast.
>
> Do you know of some good documentation or books that I can start with?
>
> Dan.
>
> On Sun, Jan 16, 2011 at 9:14 AM, Nick Hilliard <nick at foobar.org> wrote:
>> On 16/01/2011 02:30, Dan Letkeman wrote:
>>>
>>> Drops are happening even when its not under load.  Has nothing to do
>>> with bandwidth.
>>
>> Dan,
>>
>> hypothetically on a 100Mb port, if you burst your output to 200 megs for 1
>> second, then drop to zero traffic for 4 minutes 59 seconds, you will see:
>>
>> - 50% packet loss on the link
>> - a 5 minute throughput rate of 333000 bits per sec
>>
>> This is called a microburst.  I.e. a burst of traffic which goes beyond the
>> capacity of the link, but which is too short to be measured accurately by
>> your 5 minute rolling average.  Typically you'll see this on slower speed
>> lan links with bursty traffic, and it's why you're seeing relatively low
>> levels of traffic, but output drops on the interface.
>>
>> If you want to fix this problem, you have several potential workarounds:
>>
>> - increase your port speeds
>> - get a switch with bigger buffers
>> - tune the output buffers on your existing switch
>> - in your particular case, you could try fiddling with the etherchannel
>> hashing algorithm to see if it helps (it's unlikely to make the problem
>> disappear completely).
>>
>> Going back to your port channel
>>
>>> Port-channel2 is up, line protocol is up (connected)
>>>  Hardware is EtherChannel, address is 001b.d59d.7199 (bia 001b.d59d.7199)
>>>  MTU 1500 bytes, BW 200000 Kbit, DLY 100 usec,
>>>     reliability 255/255, txload 24/255, rxload 2/255
>>>  Encapsulation ARPA, loopback not set
>>>  Keepalive set (10 sec)
>>>  Full-duplex, 100Mb/s, link type is auto, media type is unknown
>>>  input flow-control is off, output flow-control is unsupported
>>>  Members in this channel: Fa0/23 Fa0/24
>>
>> Your problem is here ------> ^^^^^^^^^^^^^
>>
>> You need to upgrade your switch to a gig capable device.  You've outgrown
>> your existing equipment.
>>
>> Nick
>>
>
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