Q. How can InterMapper query a particular MIB variable?
A. There are two kinds of MIB variables: scalar values and table entries.
Scalars have a single value, such as the interface number shown above. For example, the ifNumber MIB variable of a router is a single number that represents the total number of its interfaces (ports).
Table values, on the other hand, provide the same pieces of information for different items, such as the traffic for each of a router's ports, or information about each of the TCP connections in a device.
InterMapper can read and display both scalar variables and table variables in its custom SNMP probes.
Scalar values must have a ".0" suffix in their OIDs. For example, the OID for ifNumber in MIB-II is often written as "1.3.6.1.2.1.2.1". In custom probe files, it should be represented as "1.3.6.1.2.1.2.1.0". (This ".0" is technically part of the OID - it's convenient not to write it, though.)
Table variables are generally suffixed with the index of the row. (This isn't always true: see the note below). For example, the Cisco Environment Monitoring MIB defines two variables for the input air temperature and input voltage as the first rows in each of these tables:
ciscoEnvMonTemperatureStatusValue 1.3.6.1.4.1.9.9.13.1.3.1.3
ciscoEnvMonVoltageStatusValue 1.3.6.1.4.1.9.9.13.1.2.1.3
If you add a suffix ".1" to each of these, you'll get the value of the first row; add ".2" to as a suffix, you'll get the second row, etc.
As noted above, some tables don't have a separate index column. These rows are named (their OIDs are specified by) data in the row. For example, the OID for tcpConnState row, the status of a particular TCP connection is "1.3.6.1.2.1.6.13.1.1". Its index is the source and destination IP address and port (all four values) which are appended to the tcpConnState OID. Thus, the full OID for the state of a TCP connection from 9.8.7.6 port 543 to 123.45.67.89 port 8765 would be:
1.3.6.1.2.1.6.13.1.1.9.8.7.6.543.123.45.67.89.8765
