What exactly is Secondary DNS?

Is it a good idea to use secondary DNS?

[2lame2rank]

My hosting companys two nameservers are on the same IP block, which I believe is a problem. My domain registrar offers "Secondary DNS", but I don't really understand their explanation on what it is. Does Secondary DNS just copy the records from the other nameservers? Is it a good idea/safe/recomended to use it? Thank you!

[IanTurner]

Secondary DNS just copies records from the primary DNS and serves the results to those who query.

Perfectly safe to use if the service is being supplied by a reputable company.

[himalayaswater]

Use secondary DNS for reliability of your name service.

Secondary DNS is useful if your primary dns server went down or too busy to serve request. Some browser or client may send domain request in round robin fashion.

[2lame2rank]

Secondary DNS is useful if your primary dns server went down or too busy to serve request

Thanks for your replies, the above is what I'm interested in. How does this system work exactly? Say I have three nameservers, two from my host and one from my domain provider (secondary DNS). If my host's two servers are unreachable does that mean that browsers will try each one in turn, from 1 to 3, and take the first one that replies? Or do they check any at random?. If one fails, do they try another on the list?

[IanTurner]

They try one at random and if that fails check another.

[himalayaswater]

It all depends upon os and local resolver configuration. In simple term, browser will try to check all name server before returning domain not found error. It is good idea to have secondary name server outside your data center or country.