Hello Mohammad
Yes, the internal and external interfaces belong to the border router. In this example, these interfaces are on the local device since both the master controller and the border router are the same device. However, even if the border router is a different device, the configuration would be the same. Here’s an example for you.
Imagine the master controller is R2 from the lesson. Then, the configurations would be like so:
R2 configuration (master controller)
R2(config)#oer master
R2(config-oer-mc)#border 192.168.123.3 key-chain OER
R2(config-oer-mc-br)#interface fastethernet 0/0 internal
R2(config-oer-mc-br)#interface serial1/1 external
R2(config-oer-mc-br)#interface serial1/0 external
R2(config-oer-mc-br)#exit
R2(config-oer-mc)#exit
R3 configuration (border router)
R3(config)# oer border
R3(config-oer-br)# local FastEthernet0/0
R3(config-oer-br)# master 192.168.123.2 key-chain OER
R3(config-oer-br)# end
In this case, you still configure the internal and external interfaces of the border router on the master controller, even though those interfaces are not locally on the device. The border router configuration consists of specifying the master as well as the source interface to the master controller.
For more information, take a look at this Cisco command reference:
https://www.cisco.com/c/en/us/td/docs/ios/oer/command/reference/oer_book/oer_02.html#wp1031408
I hope this has been helpful!
Laz