[mrtg] Re: Hand up to Microsoft.

Alex van den Bogaerdt alex at ergens.op.het.net
Thu Dec 5 17:19:23 MET 2002

On Thu, Dec 05, 2002 at 10:51:49AM -0500, Kevin Stone wrote:
> I've had it happen before after a reboot.  Since I've seen it happen on
> other systems and devices I'm not sure if it's a Microsoft or an SNMP
> design problem.

The instance is required to stay the same for at least the time
the SNMP process is running.  This means that in theory it is
allowed after a reboot to assign random instance numbers.

In reality I think there's a huge difference between changing
instances each time a switch is booted (several months or years)
and each time a host is booted (in some cases more than once a

Last but not least:  It is an SNMP problem.  It is not a design
problem per se, the implementation doesn't handle the changing
instances (but should do so, depending on the uptime).  So many
SNMP clients don't care or even know about the changing instances
that for instance Cisco provides an option to have permanent
instance numbers.

MRTG by the way handles this problem where possible.  If you query
a router for the interface with IP address "a.b.c.d" the instance
may change.  Of course,  if you specify to query the router for
instance "x" MRTG does just what you tell it to do.


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