You manage the Cisco ACI fabric in the following exhibit:

S2 has just been discovered by APIC1. Only one leaf switch has been discovered and registered with APIC1 so far. No other spine switches have been discovered.
Which of the following switches will most likely be discovered next?

A. L5

B. S1

C. L1, L2, and L3
D. L4 and L6
E. L1, L2, L3, L4, and L6

Explanation:

Most likely, S1 will be discovered next because of the way that Cisco Application Policy Infrastructure Controllers (APICs) discover and register leaf and spine switches in a Cisco Application Centric Infrastructure (ACI) fabric. In this scenario, APIC1 Has just discovered S2. This means that L5, the leaf switch to which APIC1 is directly connected, has already been discovered as well. L5 is directly connected to both spine switches in the topology. Because S2 was just discovered and no other spine switches have yet been discovered, it is likely that APIC1 will next discover S1.
APIC1 will not discover L5 next, because APIC1 has already discovered and registered L5. When a Cisco APIC begins the switch discovery process, it first detects only the leaf switch to which it is connected. After that leaf switch is registered, the APIC discovers each of the spine switches to which the leaf switch is connected. Spine switches do not automatically register with the APIC. When a spine switch is registered with the APIC, the APIC will discover all the leaf switches that are connected to that spine switch. Therefore, APIC1 will not discover all the leaf switches in this scenario until each spine switch is registered with the APIC.
APIC1 will not discover L1, L2, L3, L4, or L6 next. All these switches are leaf switches that are directly connected to both S1 and S2. Although APIC1 in this scenario will eventually discover L1, L2, and L3, it will first discover all the spine switches to which L5 is directly connected.

Reference:

Cisco: Fabric Initialization and Switch Discovery: Switch Discovery