Refer to the exhibit. A tenant is configured with a single L3Out and a single-homed link to the core router called Core-1. An engineer must add a second link to the L3Out that connects to the Core-2…

Adding a second OSPF interface profile (if OSPF is used) is a protocol configuration step, but the fundamental action for physical connectivity is defining the path itself.

Adding an 'interface to the external domain' is not the precise ACI terminology; the correct approach involves adding a 'path' to an existing logical interface profile.

In Cisco ACI, an L3Out defines the fabric's connection to an external routing domain. To introduce a second physical link to a different core router (Core-2) for an existing L3Out, a new path must be added to the logical interface profile within that L3Out. This path specifies the new physical interface on the leaf switch, its encapsulation, and other link-specific details, effectively extending the L3Out's physical connectivity.

Adding a second subnet to the external EPG is used for advertising or consuming additional IP address prefixes, not for establishing the underlying physical link of the L3Out.