SNMP ifIndex persistence

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Your network monitoring application, such as MRTG, is configured to monitor a specific interface. Since there are more than one interface on a device, monitoring will be based on the specific interface's assigned ifIndex.

However, after a hard device reboot, the ifIndex has changed, causing problems with MRTG reporting. This may occur intermittently; sometimes, ifIndex are the same after a reboot, sometimes they change (especially after adding logical interfaces and/or FastEthernet or GigabitEthernet modules).

This has been addressed in RFC2863 (updatint the ifIndex definition under RFC1213). As such, Cisco introduced SNMP ifIndex persistence; simply put, the current ifIndex assigned to the interface will be the permanent.

Configuration:

SNMP ifIndex persistence can be applied globally to all interfaces, or only to specific ones. The syntax are as follows:

 MyDevice(config)# snmp-server ifindex persist
 MyDevice(config-if)# snmp ifindex persist

Note: The actual syntax may vary depending on the IOS version; older versions use snmp-server ifindex persist instead of just snmp ifindex persist.

After that, you need to save the configuration via write mem or copy run start.

Verification

To retain the ifIndex values, like the configuration and vlan information in some older devices, it will be saved in the nvram. Check the NVRAM for a file called ifIndex-table.

MyDevice#dir nvram:ifIndex-table
Directory of nvram:/ifIndex-table

   3 -rw- 6712 <no date> ifIndex-table

1964024 bytes total (1752173 bytes free)
MyDevice#


Once that is configured, the changing ifIndex problem should be resolved and reporting tools such as MRTG will have no problems monitoring the correct interface.

References:
Interface Index (ifIndex) Persistence