This read-write value configures the Security Parameter Index (SPI) tied to the Mobile Node-AAA shared secret, identifying the security association used for AAA-side mobile-IP authentication. An admin must match this SPI to the value the AAA server expects, since a mismatch here would break mobile-IP AAA authentication independently of any home-agent-side settings. For example, after rotating the MN-AAA shared secret with the carrier, the admin would need to update this SPI value to match if the carrier also changed the associated index.