The DHCP server sends the update information to the DNM server, not the primary DNS server. The primary DNS
server retrieves this information from DNM via a zone transfer.
The DHCP server will keep an open connection between it and the DNM server so if that connection is not up the DHCP server will attempt to re-connect. If gives up after (I believe) 4 unsuccessful attempts. So if DNM is not accessible the DNS update info never gets sent.
The support for forwarding dynamic updates to DNS like a windows based DHCP server does not depend
on the IOS. You only need to use the following commands:
Router#config t
router(config)#int e0 (or the interface you are using)
#ip helper-address (here you have to place the address of the
external DCHP, the router must be able to ping this address)
#ip directed-broadcast