You are the DNS administrator for TXGlobal, which is headquartered in Dallas and has branch offices in Austin, Houston, San Antonio and El Paso.
The headquarters location hosts the parent domain, txglobal.com. Each branch office has been delegated a child domain.
Austin hosts austin.txglobal.com, Houston hosts houston.txglobal.com, San Antonio hosts sanantonio.txglobal.com, and El Paso hosts elpaso.txglobal.com.
The primary DNS server in Dallas is named TXDNS. The primary DNS servers in the branch offices are named AUSDNS, HOUDNS, SADNS and EPDNS.
The secondary DNS servers in the branch offices are named AUSDNS-2, HOUDNS2,SADNS-2, and EPDNS-2.
To increase fault tolerance, you want to add another secondary DNS server in each location.
The new DNS servers will be named AUSDNS-3, HOUDNS-3, SADNS-3, and EPDNS-3. You want TXDNS to be aware that the new servers are authoritative for their respective zones.
Which server or servers should host one or more stub zones?