There are several things that must be done for this to work. The first important thing, as you mention, is to configure ip helper-address on the LAN interface of the router on which DHCP requests will be received. It is also necessary that the DHCP traffic be interesting traffic. And it is necessary that there be routing logic (static routes or whatever) so that the router will know to forward over the ISDN to get to the server. And the server side must have configuration to return the DHCP traffic (correct interface configuration, interesting traffic, routing logic that will forward traffic to the remote router over the ISDN).
I do not believe that the issue about timing of the DHCP over ISDN is a serious issue. The original DHCP request may or may not be completed in a timely fashion. Depending on how much traffic is interesting the link may already be up when the DHCP request is made. And if the first request does time out, the PC should issue another request. And when the next request is received the ISDN should be up and the connection established. Though I do believe that he makes an interesting suggestion about configuring DHCP on the router instead of using the remote server.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...