Which two statements about tenant bridge domain Layer 2 unknown unicast forwarding modes are true? (Choose two.)

Question

Options

• AIf forwarding mode is flood, unknown Layer 2 unicast is sent to all Layer 2 ports in the bridge domain.
• BIf forwarding mode is proxy, unknown Layer 2 unicast packets are sent to border leaf proxy for forwarding.
• CBy default, multicast traffic is always flooded within the bridge domain VLAN.
• DLayer 2 unknown unicast can be set to flood or proxy, the default is flood.
• EIf border leaf proxy cannot resolve the destination, packet is flooded.

• A
  92% (23)
• B
  4% (1)
• E
  4% (1)

Explanation

Option A is correct because in ACI bridge domain flood mode, the forwarding behavior mirrors traditional switching: any unknown Layer 2 unicast frame (destination MAC not in the endpoint table) is flooded out all L2 ports within that bridge domain, ensuring reachability at the cost of extra traffic.

Why the distractors are wrong:

• B is incorrect because in proxy mode, unknown unicast packets are forwarded to the spine proxy (not the border leaf proxy) - the spine holds a global endpoint table and attempts MAC resolution there.
• C is a scope distractor - multicast handling is a separate bridge domain setting and is not always flooded by default; it depends on the multicast and ARP flooding configuration.
• D is incorrect on the default: in ACI, the default L2 unknown unicast forwarding mode is proxy (hardware proxy), not flood.
• E is incorrect because when the spine proxy cannot resolve the destination in proxy mode, the packet is typically dropped, not flooded - this is a key behavioral difference from flood mode.

Note: The question asks for two correct answers, but only A is listed in the provided key. Based on ACI documentation, A is definitively correct; review your source material for the intended second answer, as B/D contain partial truths that vary by exam version.

Memory tip: Think of the modes by their names - flood = fire hose (spray everywhere), proxy = front desk (ask the spine to look it up). If the front desk can't find the guest, they turn them away - they don't announce it over the intercom.

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