- A
The 'L3 Unknown Multicast Flooding' is set to flood.
Why wrong: This setting affects multicast forwarding, not unicast communication.
- B
The 'ARP Flooding' is enabled.
In hardware proxy mode, ARP flooding should be disabled to enable proxy ARP. If enabled, the leaf will flood ARP requests and proxy behavior may not function, potentially breaking communication.
- C
The contracts are unidirectional.
Why wrong: Contracts can be unidirectional by design; that would not prevent one-way communication, but two-way would need two contracts. However, even with unidirectional, traffic should flow in the allowed direction.
- D
The bridge domains are in different subnets.
Why wrong: Different subnets require routing, which is enabled by 'Unicast Routing'. ACI can route between BDs in the same VRF via the contract.
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
In a Cisco ACI fabric, a tenant has multiple bridge domains in the same VRF all with 'Unicast Routing' enabled and hardware proxy mode. However, endpoints in different BDs within the same VRF cannot communicate even with a contract. What is a possible reason?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The 'ARP Flooding' is enabled.
When 'Unicast Routing' is enabled on a bridge domain (BD) in hardware proxy mode, the ACI fabric relies on the endpoint database to forward traffic between BDs within the same VRF. For inter-BD communication, the source BD must learn the destination endpoint's MAC address via ARP. If 'ARP Flooding' is disabled (the default when Unicast Routing is enabled), the fabric does not flood ARP requests to remote BDs; instead, it expects the ARP request to be resolved by the COOP database. However, in hardware proxy mode, the fabric does not automatically proxy ARP for endpoints in different BDs, so ARP requests are dropped, preventing communication even with a contract. Enabling 'ARP Flooding' allows ARP requests to flood across BDs, enabling endpoint discovery and thus inter-BD communication.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The 'L3 Unknown Multicast Flooding' is set to flood.
Why it's wrong here
This setting affects multicast forwarding, not unicast communication.
- ✓
The 'ARP Flooding' is enabled.
Why this is correct
In hardware proxy mode, ARP flooding should be disabled to enable proxy ARP. If enabled, the leaf will flood ARP requests and proxy behavior may not function, potentially breaking communication.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The contracts are unidirectional.
Why it's wrong here
Contracts can be unidirectional by design; that would not prevent one-way communication, but two-way would need two contracts. However, even with unidirectional, traffic should flow in the allowed direction.
- ✗
The bridge domains are in different subnets.
Why it's wrong here
Different subnets require routing, which is enabled by 'Unicast Routing'. ACI can route between BDs in the same VRF via the contract.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that enabling 'Unicast Routing' and a contract is sufficient for inter-BD communication, but the trap is that ARP flooding must also be enabled to allow endpoint discovery across bridge domains in hardware proxy mode.
Detailed technical explanation
How to think about this question
In ACI, when Unicast Routing is enabled on a BD, the fabric uses the COOP database to resolve endpoint locations. However, in hardware proxy mode, the leaf switch does not perform ARP proxy for endpoints in different BDs; it relies on the ARP request being flooded to all BDs in the VRF. If ARP Flooding is disabled, the ARP request is only sent to the local BD's subnet, so the destination endpoint in another BD never receives the ARP, and the source cannot learn the destination MAC. This is a common misconfiguration when deploying multi-BD VRFs with contracts, as engineers assume contracts alone enable communication without ensuring Layer 2 reachability for ARP.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Questions learners often ask
What does this 350-601 question test?
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'ARP Flooding' is enabled. — When 'Unicast Routing' is enabled on a bridge domain (BD) in hardware proxy mode, the ACI fabric relies on the endpoint database to forward traffic between BDs within the same VRF. For inter-BD communication, the source BD must learn the destination endpoint's MAC address via ARP. If 'ARP Flooding' is disabled (the default when Unicast Routing is enabled), the fabric does not flood ARP requests to remote BDs; instead, it expects the ARP request to be resolved by the COOP database. However, in hardware proxy mode, the fabric does not automatically proxy ARP for endpoints in different BDs, so ARP requests are dropped, preventing communication even with a contract. Enabling 'ARP Flooding' allows ARP requests to flood across BDs, enabling endpoint discovery and thus inter-BD communication.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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