Controlling Interface Numbering

Each interface in the ifTable is assigned an ifIndex number. RFC 1213 required that ifIndexes use contiguous integers and that the ifIndex be less than the value of the total number of interfaces (ifNumber). More recent RFCs—1573, 2232, and 2863—removed these restrictions to accommodate interface sublayers. The E-series router implementation of SNMP derives index numbers in 32-bit values that are unique on a given router. This numbering scheme can result in large gaps in the ifIndex.

Legacy network management software that was designed to work with RFC 1213 implementations expects contiguous integers and can fail when the software encounters large gaps in the ifIndex.

By default, the router uses a numbering scheme based on RFC 2863. For compatibility with RFC 1213, you can set up the router to use contiguous numbers and to limit the values of the ifIndex and the ifNumber.

snmp-server interfaces rfc1213

• Use to set up the interface numbering method in the IfTable to use contiguous integers, which provides compatibility with versions of SNMP that are based on RFC 1213.
• The maxIfIndex option sets the maximum value of the ifIndex field that the system will allocate.
• The maxIfNumber option sets the maximum number of interfaces allowed in the interface tables.

  Caution: Reducing the value of the maxIfIndex and/or maxIfNumber causes the router to automatically reboot to factory default settings.

• When the IfIndex and IfNumber maximums are reached, the system logs the event and ignores the creation of additional interfaces, which means that new interfaces are not visible in the interface table.
• Example

  host1(config)#snmp-server interfaces rfc1213 55000 100000

  WARNING: Execution of this command will cause all configuration settings to revert to factory defaults upon the next system reboot.

  Proceed with 'snmp interfaces rfc1213'? [confirm]

• Use the no version to return to the default method of interface numbering.
• See snmp-server interfaces rfc1213.

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