[TriLUG] Odd Network problem

[Jason Tower]

i should clarify: ethernet switches don't track mac-to-ip pairs (like 
arp does), they track mac-to-switch port pairs.  but the effect is 
similar - if either one isn't working, some packets won't make it to 
their destination.

On Sunday 13 February 2005 11:23, Jason Tower wrote:
> when the nagios service stops working (can't ping), can the OS still
> ping the remote host?  that's the first step - eliminate nagios
> itself as the culprit.
>
> assuming that isn't the issue, it sounds like an arp problem of some
> sort.  either the nagios host isn't keeping track of mac-to-ip
> mappings or one of the switches on the network isn't.
>
> try running 'arp -a' on the nagios box, see if the arp table looks
> correct when a remote host can't be pinged, if it does make sure the
> mac addy is correct for the corresponsing ip.  if all that checks
> out, one of the switches is probably the culprit.  if they're managed
> switches you should be able to check out the mac table, otherwise
> there's very little you can do except swap it out and see if that
> helps.
>
> are you doing any cloning of systems there?  mac addresses can be
> spoofed, either on purpose or accidentally, and if a switch or host
> sees two IPs with the same mac it might get confused.
>
> good luck - jason
>
> On Sunday 13 February 2005 01:06, Chris Knowles wrote:
> > Got a weird one.
> >
> > (Oh, regarding that crashing box, further investigation pointed at
> > the motherboard as a culprit.)
> >
> > I've got a Nagios server in place that's been happily warning us of
> > doom and gloom for over a year.  It's one of the great success
> > stories for Linux at our company.
> >
> > Until now.
> >
> > Starting this morning, it has been randomly unable to ping various
> > boxes on our network.  That is, until you ping the nagios server
> > from the "unpingable" server.   Then Nagios can ping that server
> > all it wants.
> >
> > This is all local network, no routing involved.
> >
> > Any idears as to what could be causing this?  (This is a simple
> > switched network, and other than this seems to be working fine.)
> >
> > Any help is appreciated.
> >
> > CJK