This read-write value configures the Security Parameter Index (SPI) associated with the Mobile Node-Home Agent shared secret, used to identify which security association to apply for mobile-IP authentication. An admin must configure this SPI to match the value expected by the home agent, since mobile-IP registration relies on both sides agreeing on which security association (and therefore which shared secret) is in use. For example, a mismatched SPI value here compared to the home agent's configuration would cause mobile-IP registration requests to be rejected even if the shared secret itself is correct.