SNMP information exchange

The type of information exchanged between the SNMP Manager and the SNMP Agent is defined by a Management Information Base (MIB).The most common MIB is MIB-II (or MIB-2) which is related to network management, and defines the number of octets (groups of 8 bits) sent or received on a particular physical interface.

An Object Identifier (OID) is an element of an MIB. For example, the OIDs for the number of octets sent or received are:

.iso.org.dod.internet.mgmt.mib-
2.interfaces.ifTable.ifEntry.ifInOctets
.iso.org.dod.internet.mgmt.mib-
2.interfaces.ifTable.ifEntry.ifOutOctets

Because the use of MIB-II is widely prevalent, the above are often abbreviated as:

interfaces.ifTable.ifEntry.ifInOctets
interfaces.ifTable.ifEntry.ifOutOctets

and the numerical representations of these two OIDs are:

.1.3.6.1.2.1.2.2.1.10
.1.3.6.1.2.1.2.2.1.16

A particular occurrence of an OID is called an instance. This instance number is added to the end of the OID. Continuing with the examples above, you can see that the number of octets received on the first physical interface is given by

interfaces.ifTable.ifEntry.ifInOctets.1
.1.3.6.1.2.1.2.2.1.10.1

For OIDs where there is only a single occurrence, a zero is used. For example,

system.sysUptime

is the time since the network management portion of the system was last reinitialized. Thus, the only instance of system.sysUptime is

system.sysUptime.0