Cisco Systems DL-2159-05 manual Subnet Map Exchange, Home Agent Subnet Mask

Chapter 6 Configuring Proxy Mobile IP

Introduction to Mobility in IP

client devices. Rather than waiting for agent advertisements, an access point can send out an agent solicitation. This solicitation forces any agents on the network to immediately send an agent advertisement.

When an access point determines that a client device is connected to a foreign network, it acquires a care-of address for the visiting client. The care-of address is an IP address of a foreign agent that has an interface on the network being visited by a client device. An access point can share this address among many visiting client devices.

When the visiting client associates to an access point, the access point compares the client’s IP address with that of its own IP network information and detects that the client is a visitor from another network. The access point then begins the registration. However, before the access point can begin the registration process on behalf of the visiting client, it must have the home agent IP address of the visiting client, which it gets from a subnet map table.

Subnet Map Exchange

Each access point with proxy Mobile IP enabled maintains a subnet map table. The subnet map table consists of a list of home agent IP addresses and their subnet masks. Table 6-1is an example of a subnet map table.

Table 6-1 Example of a Subnet Map Table

Home Agent	Subnet Mask
10.10.10.1	255.255.255.0
10.10.4.2	255.255.255.0
10.3.4.4	255.255.255.248
10.12.1.1	255.255.0.0

Access points use the subnet map table to determine the IP address of the visiting client’s home agent. When an access point boots up or when proxy Mobile IP is first enabled on an access point, it obtains its own home agent information using the agent discovery mechanism. It sends this information to another access point called an authoritative access point (AAP). The AAP can be any access point on the network that the network administrator chooses. The AAP is responsible for maintaining the latest subnet map table.

When the AAP receives the new information, it replies to the access point with a copy of the latest subnet map table. The new access point now has the latest subnet map table locally and it is ready to perform proxy Mobile IP for visiting clients. Having the subnet map table locally helps the access point do a quick lookup for the home agent information. Meanwhile, the AAP adds the new access point to its list of access points and the home agent information to its subnet map table. The AAP then updates all the other access points with this additional piece of information.

You can designate up to three AAPs on your wireless LAN. If an access point fails to reach the first AAP, it tries the next configured AAP. The AAPs compare their subnet map tables periodically to make sure they have the same subnet map table. If the AAP detects that there are no more access points for a particular home agent, it sends an invalid registration packet with a bad SPI and group key using the broadcast address of the home agent subnet to determine if the home agent is still active. If the home agent responds, the AAP keeps the home agent entry in the subnet map table even though there are no access points in the home agent's subnet. This process supports client devices that have already roamed to foreign networks. If the home agent does not respond, the AAP deletes the home agent entry from the subnet map table.

Cisco Aironet 1200 Series Access Point Software Configuration Guide

OL-2159-05 6-9