[c-nsp] R: Old mystery... receive vs transmit discards...

[Brian Turnbow]

HI,

> Oggetto: [c-nsp] Old mystery... receive vs transmit discards...
> 
> Over the years I've noticed the network monitors pointing out various of our
> lower-end Catalyst switches (29xx, 35xx, 37xx) reporting transmit discards or
> receive discards.  Since we have some gig uplinks on some
> 10/100 switches, obviously some of this is to be expected.
> 
> As time has gone by, we have gig uplinks to most all of our switches, and
> there are "more" discards as a result.
> 
> My curiosity is over some switches showing "transmit" discards on the
> 10/100 ports, while others are showing "receive" discards on the gig uplinks.
> Most of our traffic is downstream, so I'd write this off to the 10/100s simply
> not keeping up.
> 
> But what is the "difference" between the two?  I'm guessing the receive
> discards are when the switch overall buffer capacity has been reached and
> there is no space to store additional incoming packets.  But the transmits?
> Are there a "fixed" number of buffers per port, after which they register as
> transmit discards; or are the buffers truly shared? 
> Not that I can fix the problem, just trying to get a better understanding :)  Are
> "transmit" discards better than "received" ones?

Transmit discards are worse. They have occupied resources and just when they should be transmitted there is no outgoing buffer and they are dropped.
Incoming discards can be buffer/queue related but also traffic that the switch does not need to handles in some cases (imagine a hub before the switch for example).
They are dropped before getting processed and transmitted to the outgoing port so do not  occupy resources unnecessarily..
On different models there are different buffers/qos models that may or may not be tunable/configurable to help ( a little) and some commands to help determine if you have buffer discards
Show interface , show controller ethernt etc.

Here is a good place to read up a little 
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008015bfd6.shtml#outlost


HTH

Brian


> (Again, bearing in mind the traffic is primarily downstream; if it were more
> symmetric I'd expect different answers).
> 
> Thanks :)
> 
> Jeff
> 
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