Cisco 802.11 Wireless Networking Quick Reference - page 56

Roaming

The ability to wirelessly connect to the network provides a wonderful freedom for mobile users. Users can move to different locations in a building or different locations on a campus and still access the network. This freedom comes in two forms: portability and mobility.

Portability is the act of being able to access the network from anyplace in the organization. That is, if he opens his laptop in the break room and is able to connect, he has portability. The important element here is that the user has moved from one cell to another and isn't concerned with constant coverage.

Mobility, on the other hand, is concerned with constant coverage. Consider the nurse who is using a wireless IP phone at a hospital. As she moves between cells , she cannot restart the phone to get access to the new cell. She needs to roam freely .

Cisco enables roaming via its fast, secure roaming technology. This section describes fast, secure roaming and configuring APs for roaming clients .

Fast, Secure Roaming

When stations move between APs, it is necessary to reauthenticate the devices to the AP. Obviously, if someone is using latency-sensitive applications (such as a nurse, for example), the time it takes to reauthenticate becomes troublesome .

Fast, secure roaming allows LEAP-enabled clients to roam between APs without the need to bother the server. Rather, Cisco Centralized Key Management (CCKM) is used on an AP, which is set up to provide Wireless Domain Services (WDS).

Note

WDS is a collection of IOS software features for such tasks as enhancing WLAN mobility as well as radio management and client tracking.

CCKM allows the authentication to occur quickly, and bypasses the authentication server. Figure 7-8 shows this process.

Figure 7-8. Roaming Is Seamless with WDS

The WDS device keeps a list of credentials for CCKM clients that are part of the WLAN. When a station roams into the AP's service area, the station sends a reassociation request to the AP, which is then forwarded to the WDS device. After authenticated by the WDS, the WDS forwards the information to the AP, which then allows the connection. This seems like a lengthy process, but only two packets are sent between the station and the AP.

Configuring WDS

The following steps show the command-line interface (CLI) instructions that you can issue to your AP to configure it as a WDS AP:

Step 1.	Configure your APs or switches as WDS devices.
Step 2.	Configure your APs to use the WDS device.
Step 3.	Enable Layer 3 Mobility to an SSID.
Step 4.	Configure the authentication server to support fast, secure roaming.

You must begin in Privileged Exec mode on the AP that you wish to configure.

Step 1: Configure Your APs or Switches as WDS Devices

You can enable WDS on either the WLSM-enabled switch or the AP. When you set up WDS on an AP, here are some tips to keep in mind:

If you use an AP as the WDS device, disable the radio or use an AP that serves a small volume of clients. Client devices that associate to the WDS AP when it starts might take several minutes to authenticate.
If the WDS AP serves client devices, it can serve 30 other APs. However, if the radio is disabled, it can serve up to 60 APs.
APs that serve as repeaters cannot be used as WDS devices.

Use the following command-line instructions to configure APs as WDS devices.

AP#

configure terminal

AP(config)#

aaa new-model


AP(config)#

wlccp wds priority 200 interface bvi1


AP(config)#

wlccp authentication-server infrastructure MI5


AP(config)#

wlccp authentication-server client any fieldops


AP(config-wlccp-auth)#

ssid FelixLeiter

AP(config-wlccp-auth)#

ssid Quarrel

AP(config)#

end

This series of instructions specifies that infrastructure devices are authenticated from server group MI5. CCKM-enabled clients that use the SSIDs of FelixLeiter and Quarrel are authenticated with the server group fieldops.

The third line of code requires some explanation. Priority 200 is used to set the priority of this WDS candidate. You can configure multiple APs as WDS candidates, and when the candidate with the highest priority is taken out of service, the candidate with the next highest priority is elected. This number is a value between 1 and 255.

The fifth line allows you to specify what type of authentication to use. In this case, any has been specified. However, you could also put in EAP or LEAP, or specify your own list of authentication methods .

Step 2: Configure Your APs to Use the WDS Device

Next, you must configure the APs to use the WDS device. To do this, see the following instructions:

AP#

configure terminal

AP(config)#

wlccp ap wds ip address 10.10.10.1


AP(config)#

wlccp ap username JamesBond password 7 a$t1nm@rt1n


AP(config)#

end

Here, the AP is configured to authenticate with a specific WDS-enabled device, and it uses the username JamesBond and the password a$t1nm@rt1n. The 7 before the password means that the AP's password is encrypted. This value could be set to 0, and means the password is unencrypted.

You must use the same username and password on the AP as a client to the authentication server.

Step 3: Enable Layer 3 Mobility to an SSID

The next step is to map an SSID to a specific mobility network ID. Follow these commands:

AP#

configure terminal

AP(config)#

interface dot11radio 0

AP(config-if)#

ssid MobileNet


AP(config-if-ssid)#

mobility network-id 7


AP(config-if-ssid)#

end

Here, the SSID MobileNet is mapped to mobility network ID 7.

Step 4: Configure the Authentication Server to Support Fast, Secure Roaming

Though the action seems to take place between the clients and the APs, don't forget that your WDS device and APs must also authenticate to the authentication server. Set up the server with usernames and passwords for the APs, along with username and password for the WDS device.

Your authentication server is similar to Access Control Server (ACS) (though the AP can also act as an authentication server for up to 50 clients), and it requires you to log into ACS, and then add the name , password, and IP address of each WDS device candidate.

You would also create user entries and passwords for the APs that use the WDS candidates.

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