In multihomed mobile networks, multiple Mobile Routers can be deployed to provide fault recovery or load sharing. Also, each Mobile Router (MR) can have a bi-directional tunnel with its own Home Agent (HA). In this multihomed mobile network scenario, the neighbor root MR can replace or share the operation of another MR. Therefore, MRs should cooperate with each other to provide session continuity or load sharing. We present an authentication and registration mechanism without introducing extra signaling messages. Using the Return Routability procedure of Mobile IPv6 and the Binding Update message with the neighbor MR registration option, the MR can authenticate and register the neighbor MR to its own HA. And the HA can use this registered neighbor MR information to provide fault recovery or load sharing.