secondary dns updates

[netdel]

Hi ALL
I have a very simple query.  Please answer.

Updates in the secondary DNS should happen when we update Serial no of
the forward/reverse file of a particualar zone at the master or it
should have
after we update serial in master and restart the server at secondary ?

My problem is that updates dont happen in secondary unless I restart
the secondary DNS server.  Please tell is it like that or it should
happen when I simply change a serial in master.

Regards
Chandan Teckchandani
mailme at chandan.zzn.com