Each entry in this table returns the ifIndex of the IP interface
associated with the lower layer ifIndex supplied as the index to this
table.
The purpose of this table is to translate L2 interfaces to corresponding
L3 interfaces for billing applications. Since interesting billing
related statistics are kept at L3, it is often useful to know the L2-L3
relationship. Note that this table is not confined to L2-L3
relationships. Any interface sublayer can be used to access this table
to determine the relationship with the L3 interface.