I am unable to perform a MIB walk on the ifIndex. Why is this?
Any variable bindings or values with an access level of not-accessible cannot be queried directly because they are part of other variable bindings in the SNMP MIB table. The ifIndex has an access level of not-accessible. Therefore, it cannot be accessed directly because it is part of the variable bindings. However, the ifIndex can be accessed indirectly through the variable bindings.
For more information, click Understanding SNMP implementation in Junos OS.