The foreign agent plays a mostly passive role in Mobile IP registration. The foreign agent adds all mobile nodes that are registered to the visitor table. The foreign agent relays registration requests between mobile nodes and home agents. Also, when the foreign agent provides the care-of address, the foreign agent de-encapsulates datagrams for delivery to the mobile node. The foreign agent also sends periodic agent advertisement messages to advertise the presence of the foreign agent.
If home agents and foreign agents support reverse tunnels, and the mobile node requests a reverse tunnel, the foreign agent then tunnels all the packets from the mobile node to the home agent. The home agent then sends the packets to the correspondent node. This process is the reverse of the home agent tunneling all of the mobile node's packets to the foreign agent for delivery to the mobile node. A foreign agent that supports reverse tunnels advertises that the reverse tunnel is supported for registration. Because of the local policy, the foreign agent can deny a registration request when the reverse tunnel flag is not set. The foreign agent can only distinguish multiple mobile nodes with the same (private) IP address when these mobile nodes are visiting different interfaces on the foreign agent. In the forward tunnel situation, the foreign agent distinguishes between multiple mobile nodes that share the same private addresses by looking at the incoming tunnel interface. The incoming tunnel interface maps to a unique home agent address.