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 RFC 1573, which is later superceded by RFC 2233, introduced the concept of sparse tables for sub-interfaces.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