This read-write field specifies an outgoing interface to use as a policy-based-routing action, an alternative to routing matched traffic via a next-hop IP address. Admins care because forcing traffic out a specific interface, such as a secondary WAN link, is a common PBR technique for traffic engineering or load distribution. For example, an admin sets this to the ifIndex of a backup WAN interface so matched packets egress that link regardless of what the routing table would otherwise choose.