no response to the EAP Response Identity packet

Hi all,

I'm configuring a network access control (NAC) using a Windows 2008 Active Directory for my clients and a Cisco 2950 switch.Every thing seems to work excepting one thing. When i connect a host  i  have this error :no response to the EAP Response Identity packet.

Is someone can help me to solve the problem or have already meet the same problem?

Thanks