Cyonix said:
It's a small to medium network with about 150 - 250 users. It has 2 domain controllers, both with dns. One of these domain controllers has an external dns server added to its NIC secondary dns servers as well as the two internal dns servers.

My complaint to the network manager was that the external dns server could cause replication issues if the domain controller for some reason falls back to this external dns server.

The more senior tech says that this is standard practice. To me this sounds very odd as Active Directory relies on dns to know who to talk to, so if the domain controller falls back to this external dns server the network is going to stop working.

note, the external dns server is the ISP's dns server.
"It's a small to medium network with about 150 - 250 users. It has 2 domain controllers, both with dns. One of these domain controllers has an external dns server added to its NIC secondary dns servers as well as the two internal dns servers."

In this case you want all clients and servers to only have primary/secondary dns server point to your domain DNS servers.  Your dns zones then have "Forwarder(s)" setup to resolve external zones.  I would remove the external dns server addresses from your server NIC config and all clients.  Your dns server(s) then resolve all addresses for clients and servers (and external addresses by dns forwarding and caching).

Clients --- |
                   | <---->Internal DNS <---->Forwards unknown<------>ISPDNS 
Servers --- |