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

[Jeff Kell]

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

Thanks :)

Jeff