How to Prevent Unknown Unicast Flooding in a Cisco ACI Bridge Domain?
An engineer must configure a Cisco ACI bridge domain to prevent flooding from unknown unicast frames. Which action must be configured in Cisco ACI to accomplish this goal?
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
Tests Cisco ACI bridge-domain L2 unknown unicast behavior, where the trap is confusing flooding controls with endpoint-learning or encapsulation options.
Cisco ACI bridge domains flood unknown unicast by default, and this page explains why switching the bridge domain to optimized (hardware-proxy) mode is the correct way to prevent that flooding. It establishes D as the answer over Flood in Encapsulation, Clear Remote MAC Entries, and Disable Remote EP Learn.
Selecting Flood in Encapsulation (A), because it sounds like a flooding-related control but actually preserves the default unknown-unicast flooding behavior.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
The question asks how to prevent flooding from unknown unicast frames in a Cisco ACI bridge domain. ACI bridge domains have an L2 Unknown Unicast setting with two modes: Flood and Hardware Proxy (often described as optimized mode). In optimized/hardware-proxy mode, the leaf switch does not flood unknown unicast across the bridge domain's multicast tree (GIPo); instead, it sends the frame to the spine proxy, which reduces or eliminates unknown unicast flooding. Option D matches this exact configuration, and the community consensus among voters and commenters places optimized mode as the intended fix. As samfab noted, the bridge domain can operate in flood mode or in "optimized mode that eliminates flooding for these frames." Marinheiro also linked the Cisco APIC Layer 2 Configuration Guide while selecting D.Why the Other Options Are Wrong
Option A, Flood in Encapsulation, is effectively the opposite of the goal: it keeps unknown unicast flooding in the encapsulation and uses the bridge domain multicast tree to flood frames. Option B, Clear Remote MAC Entries, is a maintenance action that clears dynamically learned remote MAC addresses; it does not change unknown-unicast forwarding behavior. Option C, Disable Remote EP Learn, is a fabric-wide endpoint-learning control that stops remote endpoint learning, but it does not prevent unknown unicast flooding in a bridge domain. None of these settings directly addresses the L2 Unknown Unicast mode that must be changed to optimized/hardware proxy.Community Comment Notes
Commenters overwhelmingly chose D, with Marinheiro supplying the Cisco APIC Layer 2 Configuration Guide link as supporting documentation. samfab explained the distinction clearly, stating that a bridge domain can use flood mode or "optimized mode that eliminates flooding for these frames," and tied optimized mode to hardware-proxy behavior. azizislam left no technical comment, so there is no contrary technical argument to weigh. Overall the visible community discussion aligns with the vendor-documented ACI behavior rather than suggesting an alternative answer.Official Reference
Exam Strategy
In ACI questions, translate GUI labels into forwarding behavior: 'optimized' or 'hardware proxy' suppresses unknown-unicast flooding, while 'flood' preserves it. If an option mentions remote MAC entries or remote EP learning, treat it as a distractor unless the question is specifically about endpoint learning or MAC clearing.
Frequently Asked Questions
Why does optimized mode stop unknown unicast flooding in a Cisco ACI bridge domain?
In optimized (hardware-proxy) mode, the leaf sends unknown unicast to the spine proxy instead of flooding the BD's multicast tree (GIPo), so frames are not flooded everywhere.
Is 'Flood in Encapsulation' the same as disabling unknown unicast flooding?
No. Flood in Encapsulation keeps the default flooding behavior and sends unknown unicast over the BD multicast tree; it does not prevent flooding.