This object identifies the protocol layer over
which this network interface operates. If this
network interface operates over the data-link
layer, then the value of this object refers to an
instance of ifIndex [RFC1573]. If this network interface
operates over an IPv4 interface, the value of this
object refers to an instance of ipAdEntAddr [RFC1213].
If this network interface operates over another
IPv6 interface, the value of this object refers to
an instance of ipv6IfIndex. If this network
interface is not currently operating over an active
protocol layer, then the value of this object
should be set to the OBJECT ID { 0 0 }.
This object is obsolete. The IF-STACK-TABLE may
be used to express relationships between interfaces.