Now let's talk about object identifiers.
Now the actual technical definition of an object
identifier is what you see on the screen.
It's an extensively used identification mechanism jointly developed by
ITU-T and ISO/IEC for naming any type of object,
concept,
or thing with a globally unambiguous name which requires a persistent name.
Now that's an awful lot of big words, so let's break it down a little further.
Object identifiers uses the ITU X.660 standard.
It's called the OID standard if you want to look it up.
Basically,
object identifiers in the world of SNMP define a logical
entity for SNMP data and has a tree structure that you'll
see in some of the upcoming slides,
and the very first number in the OID determines which organization
controls all of the subsequent downstream items,
0 is the ITU-T, 1 is ISO, which all of our SNMP OIDs start with 1,
it actually starts with a series of numbers,
it's always the same, which we'll see here in a moment.
And two, is a joint collaboration if you will between the ISO and the ITU-T.
Now obviously,
we really only care about the ISO SNMP computing space for this so we won't be
talking about the ITU-T or the join-ISO-itu-t namespaces.
We'll only be talking about the ISO.
So this slide gives you an example of what an OID looks like,
and like we discussed, you'll notice that it starts at the very top,
which is ISO, which is OID 1, and proceeds down a tree.
Down at the bottom, you see 1.3 .6 .1 .4 .1 .9 .9 .13,
which translates to ISO.Identified - Organization.DOD .Internet .Private.
Enterprise.Cisco Systems.ciscoMgmt in the CiscoEnvMonMIB.
Now for our purposes, all of our OIDs are going to start with 1.3 .6 .1 .4 .1,
which is the private Enterprise section of the ISO MIB tree.
Now obviously, that's a lot of numbers to remember.
You might be asking well how can I be expected to remember all of these numbers.
Well, the short answer is you're not.
That's where the management information base comes in.
MIBs are basically plain text files that tell a network management system
what all the different OIDs inside that MIB are for,
how to use them, and what format they're in.
For example,
this is the ciscoEnvMonTemperatureStatusValue portion of the Cisco
environmental monitoring MIB and you'll see the syntax is Gauge32,
it is read off into degrees Celsius, and of course,
it is read-only because you can't write the number of degrees into a router,
and you'll see towards the bottom that it is actually named
ciscoEnvMonTemperatureStatusEntry and it has an entry of 3.
Now MIBs are formatted in what's called ASN.1 notation,
which again, we're not going to get really in depth with ASN.1.
But each entry builds on the previous entries.
When we look at MIBs here in just a moment,
you'll be able to see that as I go down this tree,
I can tell that interface statistics, for example,
we have an entry for errors, and discards,
and packet, and frames received and sent,
etc, and they all flow down from the interface entity.
So on the subject of MIBs,
MIBs are actually compiled into the network management system.
Each one is different,
but the process is usually where you put the MIBs in a certain
directory and it scans it every time it starts up.
You're not actually expected to remember all of the OIDs.
Obviously,
you don't want to have to drill down and find the interface statistics for a
Windows network card every time you need to add a device.
So the network management system has all of that built into it,
knows the path down to the individual interface or
whatever values you're wanting to monitor.
If you have highly specialized devices such as the Control
panel for a building sized generator,
you may have to look through the MIB and figure out what values you
want to pull back from that device so that you can then set up a
custom poller inside your network management system to poll just
that MIB because obviously,
you can't have everything in a network management system out of the box.
However,
the good news is is that so long as you can read the MIB
or contact support and ask them what OID to use to get a
specific piece of information,
you can add that into most every network management system.
Most standard MIBs are included with network management systems.
Usually, these are the ones from the ISO,
which are things like the system name, and the up time,
and the operating system version,
but also maybe large manufacturers come with nearly
every network management system, such as Intel,
or Juniper, or Cisco.
And again, if you have specialized devices from even those manufacturers,
you may still have to compile in a custom MIB or perhaps there is
more detailed information in their custom MIB,
but then what comes with the network management system.
We'll talk about all of that when we discuss installing a network management system later in the course.