Configuring Mobile IP
How Mobile IP Works
IPC-162
Cisco IOS IP Configuration Guide
If an MN determines that it is connected to a foreign link, it acquires a care-of address. Two types of
care-of addresses exist:
•FA care-of address
•Collocated care-of address
An FA care-of address is a temporary, loaned IP address that the MN acquires from the FA agent
advertisement. This type of care-of address is the exit point of the tunnel from the HA to the FA. A
collocated care-of address is an address temporarily assigned to an MN interface. This address is
assigned by DHCP or by manual configuration.
Registration
After receiving a care-of address, the MN registers this address with its HA through an exchange of
messages. The HA creates a mobility binding table that maps the home IP address of the MN to the
current care-of address of the MN. An entry in this table is called a mobility binding. The main purpose
of registration is to create, modify, or delete the mobility binding of an MN at its HA.
During registration, the MN also asks for service from the FA.
The HA advertises reachability to the home IP address of the MN, thereby attracting packets that are
destined for that address. When a device on the Internet, called a corresponding node (CN), sends a
packet to the MN, the packet is routed to the home network of the MN. The HA intercepts the packet and
tunnels it to the registered care-of address of the MN. At the care-of address, the FA extracts the packet
from the tunnel and delivers it to the MN.
If the MN is sending registration requests through a FA, the FA keeps track of all visiting MNs by
keeping a visitor list. The FA relays the registration request directly to the HA without the need for
tunneling. The FA serves as the router for all packets sent by the visiting MN.
When the MN powers down or determines that it is reconnected to its home link, it deregisters by sending
a deregistration request to the HA. The HA then reclaims the MN.
Routing
Because the major function of a Layer 3 protocol is routing, the major features of Mobile IP deal with
how to route packets to users who are mobile.
Mobile IP is a tunneling-based solution that takes advantage of the Cisco-created generic routing
encapsulation (GRE) tunneling technology and simpler IP-in-IP tunneling protocol. The traffic destined
for the MN is forwarded in a triangular manner. When the CN (a device on the Internet) sends a packet
to the MN, the HA redirects the packet by tunneling to the care-of address (current location) of the MN
on the foreign network. The FA receives the packet from the HA and forwards it locally to the MN.
However, packets sent by the MN are routed directly to the CN.
See Figure 28 for a diagram of typical packet forwarding in Mobile IP.