Why are zone tranfers failing?

[Staalenburg, Juan]

Zone transfer to a secondary name server for my primary domain is failing.  I know that it is related to firewall settings because if I open the name server up to all of IP it work fine.  I would like to limit access to my dns server to only port 53 (TCP and UDP).  But if I set my router up to do this, zone tranfers do not complete.  Can someone enlighten me! 

Thanks.