By David Davis
Monday, September 18 2006 10:20 AM
URL: http://www.zdnetasia.com/techguide/wireless/0,39044905,61953281,00.htm

IP mobility is software and hardware designed to provide network access as a user travels from network to network. Cisco calls this the ubiquitous connection.

Cellular networks already accomplish this: You can begin a cellular call in California and--in theory--you should be able to drive to New York without dropping the call (that is, as long as there are towers along the interstate highways). Granted, it usually doesn't work exactly like this, but it typically works well enough in a metropolitan area.

During that time, the call is moving from cell tower to cell tower, and the network protocols behind the scenes make your network connectivity seem seamless. Mobile IP uses the same concept but applies it to IP network devices on a LAN. The most obvious use for this is in a campus-area network.

For example, if you have a wireless PDA, you want to be able to walk from your building to another building, say 100 yards away, without losing network connectivity--or having to reconnect to the wireless network. The two buildings are on separate IP networks, and each has the 10.10.10.0/24 network.

Traditionally, your IP address needs to change when you move from building to building (i.e., network to network). You could use Dynamic Host Configuration Protocol (DHCP), but you would experience a network interruption when receiving a new address and waiting for whatever application you're using to reconnect. How can you make this process seamless without any such interruptions? Mobile IP has the answer.

What is Mobile IP?
Mobile IP is an Internet Engineering Task Force (IETF) standard, outlined in RFC 2002. A number of other IETF standards also address Mobile IP, including RFC 2003, RFC 2004, RFC 2005, and RFC 2006.

This is an industry standard—not technology that's proprietary to Cisco. This means that, in theory, there should be interoperability between Mobile IP devices (no matter the brand), assuming they support the standard. Cisco Mobile IP is a suite of software that runs on different devices, including Mobile IP that runs on IOS routers and the Cisco Mobile Client for Windows.

One Mobile IP feature that you should be familiar with is Local-Area Mobility (LAM). (A Cisco name and password are necessary to access this resource.) This features allows a device to roam from a local IP subnet to another local IP subnet, all while keeping the same IP address. This means you could walk from network to network between buildings on a large campus while using your device.

Does my router support Mobile IP?
To determine whether your router supports Mobile IP, go to the Global Configuration Mode prompt of your router, and enter the following:

ip mobile ?

Here's an example:

Router(config)#ip mobile ?
  foreign-agent         Foreign Agent services
  home-agent            Home Agent services
  host                  Grouping of one or more mobile hosts
  secure                Security association
  tunnel                Mobile IP tunnel settings
  virtual-network       IP address of virtual network 
                          containing mobile hosts

Router(config)#ip mobile

You can accomplish most Mobile IP configuration using the ip mobile command. You also use this command to enable Mobile IP routing. Here's an example:

Router(config)# router mobile

Space constraints prevent me from discussing how to configure Mobile IP in this article. For more information on how Mobile IP works and to learn how to configure it, check out in Cisco's Mobile IP IOS Configuration Guide. (A Cisco name and password are necessary to access this resource.)

Will Mobile IP work for my organization?
As you might imagine, configuring Mobile IP and preventing users from experiencing any interruptions when roaming from LAN to WLAN to DSL to coffee shop can still be somewhat problematic. Even with Cisco hardware and software supporting Mobile IP, I wouldn't bet my job that I could use Mobile IP to provide seamless roaming (that ubiquitous connectivity) for my CIO.

In practice, it's better to start out small. I suggest configuring Mobile IP on Cisco routers so your users can "roam" from one wireless LAN subnet to another. In my experience, this is the best use for Mobile IP. This way, users can use PDAs when walking from floor to floor or building to building--in other words, subnet to subnet.

The Cisco IOS mobility software allows you to roam across subnets on a LAN or wireless network, and it works well. But keep in mind that Mobile IP is a relatively new concept for most organizations, and many companies haven't implemented it yet.

The concept of being able to seamlessly roam from network to network is a great idea. But in today's world, I think it's still more of a concept than real-world technology.

David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.