RE: IfIndex persistance

On Thursday 17 February 2005 17:30, David Goodenough wrote:
>=20
> On Thursday 17 February 2005 17:30, Robert Story wrote:

....

> > DG> > If you bring up a tunnel to site A, what is the
> interface name?
> > DG> > If you take that tunnel down and bring up a tunnel to site B,
> > DG> > does it get the same name, or a different one?
> >
> > You didn't answer this question. Do you get 'tun0' then
> 'tun1', or do
> > you get 'tun0' for both?
>=20
> You would get tun0 for both.

This touches upon an issue I fought with while maintianing an NMS for a
large ISP. How do you tell when an interface is a new interface versus =
and
old one returned? Consider the case of an MRTG like monitoring system. =
You
want to collect historical data on an interface. The trouble arises when =
a
router restarts its snmp agent. Ideally, the same interface will always =
get
the same ifIndex. In the case of a simple reboot, physical interfaces =
pretty
much never change ifIndexes, at least in the systems I've seen. However =
when
virtual interfaces such tunnels are added, or hardware changes, the
situation becomes quite muddy.

Tracking by ifIndex is dangerous. Tracking by interface name (ifDescr or
ifAlias) is safer, but still has many pitfalls. The problem arises from =
the
fact that the snmp agent only knows certain low level details about an
interface, but often what you want to track incorporates some high level
information. For example, in the tunnel example above, if the tunnel is =
a
vpn to a remote office that has moved to a new location, this is still =
the
same interface with some configuration changes. Most likely one would =
like
site A and site B traffic reported in the same MRTG graph, for example.
However, if site A was a contractor with whom you are no longer doing
business, and site B is a new contractor doing something completely
different for you, these are clearly distinct interfaces with distinct
histories. The key information here is not available to the snmp agent. =
How
is the system to know if tun0 to site B should have the same ifIndex as =
tun0
to site A? Probably the safest thing to do is to issue a new ifIndex =
every
time the tunnel's configuration changes and let the NMS sort it out. It
isn't clear, though, how the snmp agent is going to detect those =
changes.

I never came up with a solution I was really happy with. Tracking by IP
address sort of worked, but circuits were renumbered occasionally and =
the
history got lost. And some interfaces didn't have IP addresses, but
sub-interfaces with IP addresses.

Jeff=20



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