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 RFC 1573, which is later superceded by RFC 2233, introduced the concept of sparse tables for sub-interfaces.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.
