This writable integer identifies, by its console-assigned number, the loopback interface whose IP address is used as the source address for packets on unnumbered remote-access links built from this profile, with -1 meaning unspecified. An admin sets this so that dial-up or other unnumbered remote-access users get outbound packets sourced from a stable, always-up loopback address rather than a physical interface address that could change. For example, setting usdIpProfileLoopback to 2 tells the router to source packets from "loopback 2", matching how that loopback was configured via the console, per the MIB's own example.