CCIE RS workbook ->system management. How to get or find the ifEntry..n ? Or is it ifEntry..3 for any route model loopbacks?
One other thing to be careful about, is if the ifindex changes after reboots.
To be sure, they come up the same as expected, use this command (persist is the keyword):
snmp-server ifindex persist
I am pretty sure that this discussed in the INE CCIE training material. That is where I did most of my training.