This object indicates the ifIndex of the interface where the RPF neighbor IP address is located. When set this object to ifIndex of null0, the RPF check result will always fail. This object is meaningful when dIpmcExtStaticMrouteRpfType is 'interfaceId'.
Sources for this description
D-Link official switch MIBsD-Link MIB copies (LibreNMS/Observium Community)