BIND: Redirect all lookups to a different nameserver using NS

I'm creating a dns server for a local network, and I need to do redirection based on the domains. Also, i'm fairly new to DNS.

I have to redirect all "ia.in" domains to a different local nameserver (172.31.1.30) which already contains their entries, and all others to any external nameserver (currently i'm trying to use Google's 8.8.4.4).

I tried using forwarders, and they worked, but the DNS server is an old, slow system, so I thought it'd be better if I could use NS entries, so that the forwarding load mainly falls onto the client's side.

With the NS entries, it gives a servfail when I try to ping either any "ia.in" domain, or any external domain. Here are the relevant named.conf and zone file entries:

named.conf

zone "." IN {
    type master;
    file "out.redirect";
    allow-transfer { any; };
};

zone "ia.in" IN {
    type master;
    file "ia.zone";
    allow-transfer { any; };
//  The entries below worked
//  type forward;
//  forwarders { 172.31.1.30; };
};

out.redirect

$ORIGIN .
$TTL 1D
@       IN      SOA @ none. ( 0 1D 1H 1W 3H );
* IN NS 8.8.4.4

ia.zone

$ORIGIN ia.in.
$TTL 1D
@       IN      SOA @ none. ( 0 1D 1H 1W 3H );
ia.in IN NS 172.31.1.30

What should I change in my configuration so that this works?