What for?
It's very important to have a reliable Secondary DNS in the other network than the Primary.
OVH offers you such a service for free. Your domain need to have the following configuration:

• primary DNS: nsXXX.ovh.net (or the name of your server)
• secondary DNS: sdns1.ovh.net

You need to register your domain on sdns1.ovh.net to allow it to manage your domain.

How?
1.Log into Manager.
2.Select "Dedicated Server" section and then click the icon , select in the "Domain Management" section.
3.Click "New domain"
4. In the form that will appear insert the domain name without www at the beginning.
-if you've registered a domain name with www or other prefix, you should delete it and register once more without prefix
-if you have example.com domain name and 2 subdomains: test1.example.com and test2.example.com, you should register example.com only once.

After you fill in the form and activate the domain on sdns1.ovh.net, please modify domain's DNS to:

• nameofserver.ovh.net
• sdns1.ovh.net

If the domain is in OVH, you may do it in Manager, "Domain & DNS"