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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?

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  • Adding or removing physical interfaces
  • Adding or removing logical interfaces (vlan, dialer, tunnel, etc.)

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.)

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  • I moved around and removed WS-X4306-GB and WS-X4448-GB-RJ45 modules in WS-C4506 chassis and ifIndex values according to sh snmp mib ifmib ifindex assigned to card were not changed once they were initially(for example when module was first time inserted) assigned. Same seems to be true for tunnel or dot1q or VLAN interfaces. So in short, interface will get a new SNMP ifIndex only in case of router reload? – Martin Jun 25 '15 at 14:15
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    Yes, OIR will retain the ifIndex as long as the router isn't reloaded. This so a faulty module can be replaced. – Ricky Beam Jun 25 '15 at 20:39

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