The IP address to be used when generating packets on this interface.
This variable is complemented by vRtrIfUnnumbered. If
vRtrIfUnnumberedIf is a null-string and vRtrIfUnnumbered
is '0.0.0.0' it implies that no unnumbered interface is configured.
At any instant only one of the two members can be non-null.
A value of '255.255.255.255' implies the use of system-ip. It is
strongly suggested to use the node's system IP address as it is not
associated with a particular physical interface and is therefore always
reachable.