Quick Answer
The answer is that the L2Out is not configured to flood BUM traffic. Even though the bridge domain enables flooding for broadcast, unknown unicast, and multicast frames internally, the Layer 2 external connection (L2Out) acts as a separate policy boundary; without explicitly enabling BUM flooding on the L2Out interface profile, the ACI fabric will drop those frames at the border leaf before they can traverse the VPC to the external network. This scenario tests your understanding of how ACI decouples internal bridge domain flooding behavior from external Layer 2 extension policies—a common trap on the ENCOR 350-401 exam where candidates assume that enabling flooding in the bridge domain alone is sufficient. Remember the memory tip: “Bridge domain floods inside, L2Out floods outside—both must say ‘flood’ for BUM to cross the ride.”
This 350-401 practice question tests your understanding of virtualization. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A cloud provider uses Cisco ACI to automate provisioning of tenant networks. A new tenant requires a Layer 2 bridge domain that extends to an external Layer 2 network via a VPC. The engineer creates a bridge domain with the settings: Type: Regular, L2 Unknown Unicast: Flood, L3 Unknown Multicast Flood: Flood, and Multi-Destination Flooding: Flood. The VPC is configured as a virtual port channel. The tenant reports that broadcast traffic is not reaching the external network. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The VPC configuration does not support L2 extension.
Why wrong: VPC is valid for L2 extension.
The bridge domain is configured as proxy mode for L2 unknown unicast.
Why wrong: It is set to flood, not proxy.
The L2Out is not configured to flood BUM traffic.
L2Out must be configured with flood settings to extend flooding.
The bridge domain type should be set to 'L2 Only'.
Why wrong: Regular type supports L2 extension.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The L2Out is not configured to flood BUM traffic.
The correct answer is C because the bridge domain is configured to flood BUM (Broadcast, Unknown Unicast, and Multicast) traffic internally, but the L2Out (Layer 2 external connection) must also be explicitly configured to flood BUM traffic to the external network. Without this configuration on the L2Out, the ACI fabric will not forward broadcast or multicast frames across the VPC to the external Layer 2 network, even though the bridge domain itself permits flooding.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The VPC configuration does not support L2 extension.
Why it's wrong here
VPC is valid for L2 extension.
The bridge domain is configured as proxy mode for L2 unknown unicast.
Why it's wrong here
It is set to flood, not proxy.
The L2Out is not configured to flood BUM traffic.
Why this is correct
L2Out must be configured with flood settings to extend flooding.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The bridge domain type should be set to 'L2 Only'.
Why it's wrong here
Regular type supports L2 extension.
Common exam traps
Cisco often tests the distinction between bridge domain flood settings and L2Out flood settings, trapping candidates who assume that enabling flooding in the bridge domain automatically allows BUM traffic to reach external networks.
Detailed technical explanation
In Cisco ACI, the L2Out is a policy construct that defines how the bridge domain connects to an external Layer 2 network. Even if the bridge domain is set to flood BUM traffic, the L2Out has its own flood settings (e.g., 'flood in encap' or 'flood in vlan') that must be enabled to allow broadcast and multicast frames to egress the fabric. This is a common misconfiguration because engineers often assume that bridge domain flood settings automatically propagate to external connections, but ACI enforces separate policies for internal and external flooding to prevent unnecessary broadcast propagation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Virtualization — This question tests Virtualization — Read the scenario before looking for a memorised answer..
The correct answer is: The L2Out is not configured to flood BUM traffic. — The correct answer is C because the bridge domain is configured to flood BUM (Broadcast, Unknown Unicast, and Multicast) traffic internally, but the L2Out (Layer 2 external connection) must also be explicitly configured to flood BUM traffic to the external network. Without this configuration on the L2Out, the ACI fabric will not forward broadcast or multicast frames across the VPC to the external Layer 2 network, even though the bridge domain itself permits flooding.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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