Problem in joining DC to a existing Domain

i HAve configured a Win 2003 Domain & is running in Windows 2000 Native Mode .

Clients are able to login to domain & GPOs are working. but when i try to add another WIn 2003 server to domain as a Backup DC. am getting an error
“An Active Directory Domain Controller for the domain sensag.ind could not be contacted”

Ensure the DNS domain name is typed correctly.

Whatz causing this error ?
is there anything we have to do in DNS after installing first DC in a domain.