I know this is a very old posting, but I had the same problem and could not find an answer here or on the internet. The answer may be here under a different device or search term, but since I could not find it I dug into finding an answer. I am sure the OP doesn't not care at this point, but here is what I came up with for future searches.
First, I am looking for ISR 4300 series routers. I need temperature readings that I know they have because I can do a "show env all" and see the values. So I assume I can get them via a MIB/OID. But I could not find the exact OID to use. Even internet searches came up short. Most referenced stuff from older routers that no longer use the outdated OIDs.
What I found was the IETF standard MIB that lists various modules in the router:
entPhysicalDescr = .1.3.6.1.2.1.47.1.1.1.1.2
I downloaded and installed a MIB Walker and setup the correct ReadOnly community string on my router and "walked" the above MIB/OID to find temperature indexes of 7001, 7002, 7003...
Standard/Non-Propriety MIB
ENTITY-MIB, entPhyscialEntry, entPhyscialIndex [OctetString]
===========================================================================
Temperature indexes = 7001, 7002, 7003... Many other Environmental Indexes
.1.3.6.1.2.1.47.1.1.1.1.2.7001 [Temp: Internal]
.1.3.6.1.2.1.47.1.1.1.1.2.7002 [Temp: Outlet]
.1.3.6.1.2.1.47.1.1.1.1.2.7003 [Temp: CPU]
My MIB/OID browser did not have any value related entries for the IETF/Standard MIBS/OIDs. Only could find descriptions, software/hardware versions, etc. Digging on internet and eventually just walking the entire entPhysicalTable
(.1.3.6.1.2.1 tree), I found .99. with actual values. It turns out to be the ENTITY-SENSOR-MIB (Cisco) / ENTITY-SENSOR-MIB (IETF) tree with values. In other words, the entPhySensorValue tree.
entPhySensorType [Integer]
=================================
.1.3.6.1.2.1.99.1.1.1.1.7001 [8]
.1.3.6.1.2.1.99.1.1.1.1.7002 [8]
.1.3.6.1.2.1.99.1.1.1.1.7003 [8]
entPhySensorScale [Integer]
=================================
.1.3.6.1.2.1.99.1.1.1.2.7001 [9]
.1.3.6.1.2.1.99.1.1.1.2.7002 [9]
.1.3.6.1.2.1.99.1.1.1.2.7003 [9]
entPhySensorPreecision [Integer]
==================================
.1.3.6.1.2.1.99.1.1.1.3.7001 [0]
.1.3.6.1.2.1.99.1.1.1.3.7002 [0]
.1.3.6.1.2.1.99.1.1.1.3.7003 [0]
entPhySensorValue [Integer] <<<<<< Actual Temp Reading
=================================
.1.3.6.1.2.1.99.1.1.1.4.7001 [37]
.1.3.6.1.2.1.99.1.1.1.4.7002 [44]
.1.3.6.1.2.1.99.1.1.1.4.7003 [39]
entPhySensorOperStatus [Integer] <<<<<< Assume 1 = Normal/InRange
=================================
.1.3.6.1.2.1.99.1.1.1.5.7001 [1]
.1.3.6.1.2.1.99.1.1.1.5.7002 [1]
.1.3.6.1.2.1.99.1.1.1.5.7003 [1]
entPhySensorUnitsDisplay [OctetString] <<<<<< Unit of the Reading
======================================
.1.3.6.1.2.99.1.1.1.6.7001 [Celsius]
.1.3.6.1.2.99.1.1.1.6.7002 [Celsius]
.1.3.6.1.2.99.1.1.1.6.7003 [Celsius]
entPhySensorValueTimeStamp [timeticks]
==========================================
.1.3.6.1.2.1.99.1.1.1.7.7001 [4963 hours 47 minutes 32 seconds (1786965285)]
.1.3.6.1.2.1.99.1.1.1.7.7002 [4963 hours 47 minutes 32 seconds (1786965285)]
.1.3.6.1.2.1.99.1.1.1.7.7003 [4963 hours 47 minutes 32 seconds (1786965285)]
entPhySensorValueUpdateRate [Guage]
====================================
.1.3.6.1.2.1.99.1.1.1.8.7001 [20000]
.1.3.6.1.2.1.99.1.1.1.8.7002 [20000]
.1.3.6.1.2.1.99.1.1.1.8.7003 [20000]
Bottom line, all the values from the "show env all" were in this MIB/OID tree. Each router model will most likely be different (7001 might not be Internal Temp). You should be able to use the .1.3.6.1.2.1.47.1.1.1.1.2 tree to find the index you need and then the .1.3.6.1.2.1.99.1.1.1 tree to find the values of the sensor(s).
