OID value: 1.3.6.1.2.1.4.21.1.2
OID description:
ipRouteIfIndex OBJECT-TYPE SYNTAX INTEGER ACCESS read-write STATUS mandatory DESCRIPTION "The index value which uniquely identifies the local interface through which the next hop of this route should be reached. The interface identified by a particular value of this index is the same interface as identified by the same value of ifIndex." ::= { ipRouteEntry 2 }
See also the OID Repository website reference for 1.3.6.1.2.1.4.21.1.2