[mrtg] Re: confusion.

Graeme Fowler G.E.Fowler at lboro.ac.uk
Tue Dec 14 10:59:36 MET 1999

On 14-Dec-1999 Marc-Adrian Napoli wrote:
> we have lots of cisco's here. when i use getif to look at them SNMP
> wise i get the usual stuff up until the enterprises tree, then i get
> a whole lot of values under that.
> I'm just confused about this whole MIB thing. I thought a program
> that looked at SNMP MIB tables would get everything, why do new
> MIB's need to be compiled into the browser?

Let's go back to basics here:

You Cisco boxes run an SNMP 'agent' which is, effectively, a server.
More accurately it is an interface between SNMP clients and the values
you want to extract from the box itself, whether they be standard
(MIB-2) objects or non-standard (enterprises) objects.

An SNMP-V2 agent (like on your Cisco kit, I doubt you run V3 yet ;)
does not refer to anything with words. It purely refers to the objects
with unique numbers called identifiers (hence the term Object
IDentifier, OID).

When your SNMP client asks for information from the agent it *must*
ask for the numeric OID or it will not be able to poll the information.
The MIB files are purely a form of lookup for the SNMP client so it can
convert between the textual form of OID you may provide and the numeric
form it needs to poll the agent for.

The structure of OIDs is like a tree. In order to reference the textual
form you *must* have all the constituent parts preceding your OID
available to the client or it will not be able to carry out the
conversion. You only need one 'branch' to be missing for this to take
place, then you get numeric OIDs all the way down.

To summarise: you need *all* the MIB files for *all* the OIDs you want
to browse, *including all the intermediate ones*. Without them, all
you'll get is numbers.
And remember: the agent on the device you are polling does not have a
MIB. It just has numbers!

As an aside, if you read the MIB files (they're just text) it's quite
easy to pull together the numeric OIDs of the things you want to look
for. Just find your target, write down it's numeric reference and then
work your way back up the file to the top, noting all the previous
references on the way. It's often quicker to do this anyway!



Graeme Fowler
Network Officer, Infrastructure & Networks Group
Loughborough University Computing Services
+44 1509 228426

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