If the mobile node is registered and uses a foreign agent care-of address, the process is straight forward. The mobile node chooses its default router from among the router addresses that are advertised in the ICMP router advertisement portion of that agent advertisement. The mobile node can also consider the IP source address of the agent advertisement as another possible choice for the IP address of a default router.
The mobile node might be registered directly with the home agent by using a co-located care-of address. Then the mobile node chooses its default router from among those routers that are advertised in any ICMP router advertisement message that it receives. The network prefix of the chosen default router must match the network prefix of the care-of address of the mobile node that is externally obtained. The address might match the IP source address of the agent advertisement under the network prefix. Then the mobile node can also consider that IP source address as another possible choice for the IP address of a default router.
If the mobile node is registered, a foreign agent that supports reverse tunnels routes unicast datagrams from the mobile node to the home agent through the reverse tunnel. If the mobile node is registered with a foreign agent that provides reverse tunnel support, the mobile node must use that foreign agent as its default router.