Routing - OER master and border routers
When configuring Optimized Edge Routing (OER), a master controller and a border router must be configured. The same IOS router can play the role of both devices, or these can be configured on different devices.
When configuring the border router, internal and external interfaces are defined. These are always defined within the master controller regardless of whether or not the master controller and the border router are a single device or two different devices.
Links:
https://forum.networklessons.com/t/oer-optimized-edge-routing-basic-configuration/963/19?u=lagapides