In Cisco IOS each interface has unique ifIndex which is an integer >0(AFAIK usually starts from 1). If snmp-server ifindex persist is not used, then it is possible that ifIndexes are mapped differently than before. Under which circumstances SNMP ifIndex in Cisco IOS might change? I know that reload is one possible reason. Are there any other reasons which might cause SNMP ifIndex to change?
When Does SNMP Interface Index (ifIndex) Change in Cisco IOS?
snmp
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Best Answer
In short, anything that changes the number (or order) of IDB entries. The index starts at one and always increments. The enumeration process starts fresh when the system boots. (if persistence isn't enabled, or someone deletes the database -- it's a file in nvram.)