This object specifies the interface of the source address that
the target interface uses when originating IPv4 packets.
The value '0' cannot be written to an instance of this object.
However, it serves as a convenient value when the column is
not valid (e.g., immediately following the creation of an
instance of the object).
This column is valid only if the 'ipv4Unnumbered' bit of the
corresponding instance of cdtIfValid is '1'.