While the topology presented in the previous sections is ideal for learning Mobile IP in the lab, it requires a significant number of routers and is beyond the facilities of many small labs. The following topologies present several methods for integrating different Mobile IP components.
At the opposite end of the spectrum from the topology shown in Figure 4-1, Figure 4-2 demonstrates a lab scenario using only a single-router topology. Coupled with a pair of computers—one acting as a Mobile Node and the other acting as a CN—this solution has most of the capabilities but is more complex to understand because all the functions are integrated. One key behavior about this configuration is that IOS Mobile IP does not use tunneling when the Home Agent and FA are on the same router. Instead, the forwarding entries are updated based on the interface to which the Mobile Node is attached.
Figure 4-2 Single-Router Test Topology
Other Options for Single-Router Topology
In between these two topologies are a number of other options. Eliminating the IS and replacing the CN with a computer are good options that can have minimal impact on the testing. Without the IS, it will not be clear how the redistribution works, but Mobile IP still functions the same. You can also combine the two FAs into one FA with two interfaces, but it will not be clear when the Mobile Node changes links, because the CoA does not change using the configuration in Example 4-4. To ensure that a CoA change is seen, each interface where the Mobile Node attaches needs to be configured as a CoA and the interface-only option should be used. Normally with two CoAes configured, the FA would advertise both addresses out both interfaces. However, with the interface-only command, only the address of the physical interface is advertised.