Daren Matthews

SNMP monitoring often references interfaces using the “ifindex”, in order to solicit information.  Sometimes though, the ifindex can change or be slightly different to the actual interface number, for example:

SW-DISTRIB#show snmp mib ifmib ifindex
FastEthernet0/1: Ifindex = 1
FastEthernet0/10: Ifindex = 10
FastEthernet0/11: Ifindex = 11
FastEthernet0/12: Ifindex = 12
FastEthernet0/13: Ifindex = 21
FastEthernet0/14: Ifindex = 22
FastEthernet0/15: Ifindex = 23

[output omitted]

Also, as new physical and logical interfaces are added and removed, the ifindex may change, which will affect your monitoring policies.

So if using SNMP to target any ifindex, use snmp mib persistence:

snmp-server ifindex persist

(this saves the ifindexes into a file on the flash)

Router# dir nvram:
Directory of nvram:/

124  -rw-         953                    <no date>  startup-config
125  —-          46                    <no date>  private-config
1  -rw-         180                    <no date>  ifIndex-table