CHAPTER 1. INTRODUCTION TO SNMP 3
SNMP devices, however without a MIB you’ll have to simply guess what the
data your looking at means. In some cases this is easy, such as seeing host
names, disk usage numbers, or port status information. Other times it can b e
more difficult and a MIB is more useful. It is not unusual for some applications
to be written using strictly numeric IODs allowing the end user to avoid the
hassles of properly installing a MIB. The action of ”installing” a MIB is really
just putting it in a place where your SNMP client application can find it to
perform the translation.
SNMP can be used in 2 ways: polling and traps. Polling just means that
you write an application that sets an SNMP GET request to an agent looking
some value. This method is useful because if the device responds you get the
information you want and if the device does not respond you know there is a
problem. Polling is an active form of monitoring. On the other hand, SNMP
traps can be used for passive monitoring by configuring an agent to contact
another SNMP agent when some action occurs.
Looking at traps deeper, a network device such as a router can be configured
to send SNMP traps for certain events. For instance, you can configure Cisco
IOS to send traps either when an individual event occurs such as a linkDown
(IOS: snmp-server enable traps snmp linkdown) or when any defined trap event
happ e ns (IOS: snmp-server enable traps snmp). When a trap event o c curs, the
agent on the device will send the trap to a pre-configured destination comm only
called a trap host. The trap host will have it’s own agent running which will
accept and process the traps as they come in. The processing of these traps
are done by trap handlers. Trap Handlers can be written in any language and
are provided with information from the sent trap via STDIN. The handler can
then do whatever is appropriate to respond to the trap, such as sending email
or doing anything else you could want.
SNMP is most commonly used in conjunction with a Network Management
System (NMS). Popular NMS’s include BMC Patrol, CA Unicenter, Sun Man-
agement Console (formerly SyMon), IBM Tivoli NetView, and the world famous
HP OpenView. Even an Ope n Source NMS is now available, the aptly named
OpenNMS. The goal of a NMS is to provide a single point of monitoring and
administration of all your SNMP enabled devices. By configuring your device
agents to allow write access you can even manipulate your environment from a
single application. When an environment is architected around a NMS solution
you can be given unparalleled levels of control and visibility over your entire en-
vironment at a glance. While Net-SNMP provides all the tools you would need
to build your own NMS we won’t discuss the subject any further here. However,
bear in mind that if you think the vendor of your SNMP enabled device isn’t as
forthcoming ab out details of their agent implementation as you’d like, it’s most
likely because they would like you to simply buy their NMS or a plug-in to use
their device with one of the other popular NMS’s.
