AirPort: Update your firmware to use WPA and WDS together
To use Wi-Fi Protected Access (WPA) and Wireless Distribution System (WDS) at the same time, be sure your AirPort Express or AirPort Extreme has the latest firmware. WPA and WDS can't be used together with earlier firmware versions.
Note: To review the requirements for joining a WPA network, see document 107795. Only AirPort Extreme and AirPort Express can host a WPA or WDS network. Earlier AirPort base stations cannot. All AirPort base stations can use WEP.