Subnet Map Exchange - Cisco Aironet 1100 Series Installation And Configuration Manual

Understanding Proxy Mobile IP
The IRDP advertisements carry Mobile IP extensions that specify whether an agent is a home agent,
foreign agent, or both; its care-of address; the types of services it provides, such as reverse tunneling and
generic routing encapsulation (GRE); and the allowed registration lifetime or roaming period for visiting
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 needs to know the home agent IP address of the visiting client.
It gets the home agent's IP address by looking it up on 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.
map table.
Table 14-1 Example of a Subnet Map Table
Home Agent
10.10.10.1
10.10.4.2
10.3.4.4
10.12.1.1
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 is an access point that is responsible
for keeping 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 a deregistration packet on behalf of the broadcast address of the home
agent subnet to see 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 1100 Series Access Point Installation and Configuration Guide
14-4
Subnet Mask
255.255.255.0
255.255.255.0
255.255.255.248
255.255.0.0
Chapter 14 Configuring Proxy Mobile IP
Table 14-1 is an example of a subnet
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