[mrtg] Re: cisco - routers - access lists dropping packets - graphing?

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Wed Jun 27 22:51:22 MEST 2001

Larry Sheldon wrote:
> > I don't think the ifInDiscards will count rejected/dropped packets.
> > Discards are, unless I'm mistaking, damaged packets such as CRC errors.
> > (please tell me I'm wrong if I am).
> One of us is going to have to go look it up, I think those are bothe wrong.

Open mouth, insert foot, RTFM.  Point taken.

quote from rfc 1156 (http://the.rfceditor.org/rfc/rfc1156.txt):

"The number of inbound packets which were chosen to be discarded even
though no errors had been detected to prevent their being deliverable
to a higher-layer protocol.  One possible reason for discarding such
a packet could be to free up buffer space."

So: not applicable when errors occur and I was wrong.  Unfortunately
this doesn't tell us much about counting policy rejected packets.

> I think "discards" are packets that were not forwarded, and were not 
> accounted-for anywhere else.

Right.  Correctly received packets that are not forwarded to higher-layer
protocols does not seem to apply here so my gut feeling remains: the
packets are not counted (in ifInDiscards) if blocked by an access list.
This is because the packets will have been counted by ifInOctets.
Again: just a gut feeling and I may be wrong again.

Time and resources permitting I will setup a system where I just flood
ping some host behind a router.  I'll then measure both with and without
blocking the pings.

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