HP-UX Mobile IPv4 A.03.01 Administrator's Guide
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Chapter 3
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You do not need a Foreign Agent on a subnet if all the Mobile Nodes that visit it use
co-located Care-of Addresses. With co-located Care-of Addresses, the Mobile Node uses a
protocol such as DHCP (Dynamic Host Configuration Protocol) to determine a unique IP
address to use while attached to the foreign network.
In this example, you want the above Mobile Nodes to be able to visit (or attach to) the
subnet 16.1.1 and for other nodes (Correspondent Nodes) to communicate with the
Mobile Nodes using their original addresses (their “home” addresses) (15.1.1.20,
15.1.1.21, 15.1.1.22).
To do this, you must configure at least one node on the 16.1.1 subnet as a Foreign Agent.
For example, you could configure node 16.1.1.50 to be a Foreign Agent.
Combined Home and Foreign Agents
If you are not using AAA, a node can be both a Home Agent and a Foreign Agent. For
example, if you wanted to make the 16.1.1 subnet the home network for Mobile Nodes,
you could configure the Foreign Agent (16.1.1.50) as a combined Home and Foreign
Agent.
You could also configure the Home Agent on the 15.1.1 subnet (15.1.1.50) as a Foreign
Agent so that Mobile Nodes could visit the 15.1.1 subnet. Refer to Figure 3-2, “Combined
Home and Foreign Agents.”
Figure 3-2 Combined Home and Foreign Agents