In an overlay network model of an ACI architecture, which statement is correct?

Question

Options

• AThe Top of Rack (TOR) switch must be able to understand both the overlay and the underlay
• BAll forwarding lookups are done at the network controller.
• CThe network controller is responsible for setting up the overlay paths
• DThe underlay network must be Layer 3 only.

• A
  68% (17)
• B
  4% (1)
• C
  8% (2)
• D
  20% (5)

Explanation

In ACI's overlay model, Leaf (TOR) switches sit at the boundary between endpoints and the fabric, so they must handle VXLAN encapsulation/decapsulation (overlay) while also participating in the IP underlay routing - making them the critical dual-aware element in the architecture. Option B is wrong because ACI uses a distributed forwarding model; the APIC controller manages policy but never sits in the data plane path for forwarding lookups. Option C is incorrect because the APIC pushes policy to the leaf switches, but the leaves themselves establish and maintain the VXLAN overlay tunnels through endpoint learning - the controller doesn't directly wire up each path. Option D is wrong because while ACI's underlay happens to be Layer 3 (IS-IS), the general overlay model doesn't mandate a Layer 3-only underlay; some overlay implementations run over Layer 2 underlays.

Memory tip: Think of the TOR/Leaf switch as a "bilingual translator" - it speaks the endpoint's native language (underlay) and the fabric's tunnel language (overlay) simultaneously. If it only knew one, traffic could never cross the boundary.

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