[c-nsp] Constant output drops on etherchannel

[Dan Letkeman]

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
>