[c-nsp] Constant output drops on etherchannel

[cnsp at matthias-mueller.net]

Depending on the network and the hardware(buffer space) output drops start at 15-20% of linerate traffic. Hardware buffers on lower end switches are usually very low, so output drop happen very often.
One of the main problems leading to micro bursts (leading to buffer related output drops) is network synchronisation, e.g. systems tend to send out periodic packets at the same time (and synchronize over the time). For network protocols algorithms are implemented that avoid that synchronization, but on the application layer there are a lot of protocols, that tend to synchronize over the time (most of the time self developed protocols).
So, seeing problems at 92% linerate is normal, but with enough bad protocols running you can see the same probs at 20% linerate...

Bye,
Matthias

On Fri, 14 Jan 2011 13:18:23 -0500
Benjamin Lovell <belovell at cisco.com> wrote:

> Agreed would need some platform details but, in general, if you are seeing port get to 92% then you can be pretty much sure that you are bursting to 100% and dropping at times. 
> 
> -Ben
> 
> 
> On Jan 14, 2011, at 11:12 AM, Phil Mayers wrote:
> 
> > On 14/01/11 16:08, Dan Letkeman wrote:
> >> Hello,
> >> 
> >> I'm seeing many of our etherchannel's on different switches having output drops:
> > 
> > Platform? IOS version? Config of the interface(s) (routed, SVI, etc.)
> > 
> >>   Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 898085
> > 
> > Are you monitoring the traffic rate? Do the drops correspond to traffic bursts? Do you have QoS enabled?
> > 
> >> I also see that it usually uses one port of the etherchannel to a high
> >> degree, say 92% before it seems to push data through the other
> >> connection.
> > 
> > That's not necessarily unusual, depending on your etherchannel load balancing algorithm and traffic patterns. But you haven't really supplied enough info for people to help you.
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