Why some ifTable columns do not appear for certain interface types

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Jun 22, 2009 4:40 PM
Jun 22nd, 2009

Core issue

This is a normal behavior.

Resolution

Technologies such as ATM, frame relay and VLANs introduced a different type of interface, the virtual interface, or sub-interface.


On an ATM interface, for example, you can have several permanent virtual circuits (PVCs). Although the overall utilization of the main interface is important, the amount of traffic on individual sub-interfaces is of great interest as well. RFC 1573, which is later superceded by RFC 2233, introduced the concept of sparse tables for sub-interfaces.


Sparse tables means that a row in the ifTable for a sub-interface does not have values in columns where the objects do not apply to the sub-interface.

Cisco IOS  implemented support for subinterfaces in the ifTable starting in Cisco IOS Software Release 11.1. Frame relay and ATM LANE sub-interfaces support was added in Cisco IOS Software Release 11.1.  Support of other ATM sub-interfaces was added in Cisco IOS Software Release 12.0(1)T for Cisco 12000, 4x00/m, 72xx, and 75xx platforms.

Each subinterface is represented with two ifTable entries:

  • One for the atmSubInterface layer
  • One for the AAL5 layer

The packet and octet counters are available only for the AAL5 layer entities, since most ATM router interfaces do not support cell-layer counts.

The ifType atmSubInterface (IANA ifType number = 134) is defined for an ATM subinterface. The atmSubinterface layer is a virtual ATM layer. The Interface MIB variables that correspond to the atmSubInterface layer have the same semantics as those of the atm layer on a main, physical interface.


These conformance groups apply to the atmSubInterface layer: 

  • ifGeneralInformationGroup
  • ifFixedLengthGroup
  • ifHCFixedLengthGroup

    Refer to the Q: Some ifTable columns do not show up for certain interface types. Why? Is this a bug? section of SNMP: Frequently Asked Questions About MIB Theory and Operation for more information on why some ifTable columns do not show up for certain interface types.

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    Posted June 22, 2009 at 4:40 PM
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