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User’s Guide – version 1.6 NetWatch
40
Appendix A: SNMP
SNMP, or Simple Network Management Protocol, is a widely used standard that
allows network management software to query network devices for status and
information regardless of the type of device and the software it runs. The original
SNMP developed in 1988 is defined in RFC 1157, and the current version 3 is fully
backwards compatible. NetWatch uses SNMP version 2c when available; otherwise it
reverts to version 1.
MIBs
The SNMP standard defines a large number of properties organised into a hierarchy
that describes virtually everything about a device that a network manager might want
to know. This hierarchy is called a MIB or Management Information Base. The
SNMP standard defines a large MIB called MIB2; this describes system properties,
routing information, interface status and statistics and a lot more. A large number of
standard extension MIBs exist; the Interface MIB is used by NetWatch when
supported, since it offers better support for high capacity interfaces than the standard
MIB.
SNMP offers a way for hardware and software vendors to supply their own device-
specific information in the form of Enterprise MIBs; most vendors have their own
MIBs in addition to supporting some of the standard MIB.
OIDs
Each value defined in a MIB is referenced by its OID or Object Identifier. This is a
long sequence of numbers separated by dots. Each number in an OID describes a sub
level of the MIB hierarchy, thus the OID for the sysDescr object is 1.3.6.1.2.1.1.1,
which means
iso.org.dod.internet.mgmt.mib-2.system.sysDescr. Most of the standard MIBs you
will come across will be in the mib-2 group, and all of the enterprise MIBs are
underneath iso.org.dod.internet.private.enterprises.
Communities
The original version of SNMP used a very simple secret word scheme to protect the
information available from a device. Most SNMP-compliant devices support two
“communities”, one of which can read the value of any variable in the MIB and one
that also allows setting the values that can be configured. The community is passed to