[mrtg] Another method for finding the Interface Index

mihai at romania.eu.net mihai at romania.eu.net
Thu Nov 16 08:24:18 MET 2000

	The issue is again old plain interface index independence in MRTG (or in
any other frontend to SNMP monitoring).
	The problem was that, one cannot monitor 'volatile' interfaces using MRTG.
One good example of 'volatile' interface  is the PPP interface
pppN under Linux (N=0,1,2....). When someone implements a cheap leased line
(2 plain old modems, one shouting ATD and other ATA) and
has X leased lines connected to a Linux Box (using a multi serial board), it
needs somehow to monitor them. But if one of them goes down, and
then wakes up, the interface name will change, and the interface index too.
The solution found was to extract the interface index from:

	. = I
(ip.ipRouteTable.ipRouteEntry.ipRouteIfIndex.A.A.A.A = I)

A.A.A.A is the network address or host address from the route, and I is the
interface index of the interface on which packets will be forwarded
according to the route.
	As I've hacked this into my MRTG, I've extended the target def syntax.
%A.A.A.A works somehow like /A.A.A.A but using as a table the OID stated

All my best,
        Sandu Mihai - KPNQwest Romania Network Engineer

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