I need to monitor the temperature of some Cisco equipment via SNMP. Using this guide as a reference I was able configure Nagios to monitor the ambient (inlet) temperature in our data center.

First you'll need to find the sensor's ID number based on the description here:

snmpwalk -v 2c -On -c public router.domain.com .1.3.6.1.2.1.47.1.1.1.1.2

You should get a long list of various parts of your router. The last section of the OID is the ID of that particular sensor. Usually they are in the thousands. After that you look at another section of the SNMP tree to gather the actual temperature:

snmpget -v 2c -On -c public router.domain.com .1.3.6.1.4.1.9.9.91.1.1.1.1.4.XXXX

or view all of the sensors:

snmpwalk -v 2c -On -c public router.domain.com .1.3.6.1.4.1.9.9.91.1.1.1.1.4