Hi Udo,
Discovery state indicates this interface is not in use by any EPGs, infra services (connected to APIC), or fabric services (connected to spine). In other words, there is neither configuration nor policy pushed to this interface. If you were to log into the switch CLI and run show interface eth x/y or show interface eth x/y switchport or show ip interface eth x/y then I suspect there are no VLANs allowed on the interface (if L2 port) or no IP address assigned to the interface (if routed port). There is a two step process to take the interface out of discovery state.
1. Access Policies
In the APIC GUI, go to Fabric > Access Policies and configure policies for this interface. This includes switch profile > interface profile > interface policy group > attachable access entity profile > physical, vmm, or L3 domain > VLAN pool. This is the prerequisite configuration which prepares the interface for EPG deployment.
2. Assign the interface path to an EPG
Go the the Tenant tab and associate the interface to either an EPG or external routed network (L3 out). Based on your initial post it sounds like your goal is to assign IP addresses to the interfaces (routed ports). In this case you would want to configure an L3 out. Under Tenant > Networking > External Routed Network > Node Profile > Interface Profile is where the interface path is configured (e.g., Leaf 101 > Eth1/14).
If both steps are configured then make sure that the L3 out has an External Netwok Instance Profile (External EPG). This is configured under Tenant > Networking > External Routed Network > Networks. I would also recommend checking the external routed network pages for any faults.
-JW
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