Which Bridge Domain Setting Enables ACI Source IP Learning?
A Cisco ACI leaf switch learns the source IP address of a packet that enters the front panel port of the switch. Which bridge domain setting is used?
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
The exam tests the bridge domain's Unicast Routing option as the trigger for ACI IP learning; the trap is confusing it with ARP flooding or L2 unknown unicast hardware proxy, which control flooding rather than source IP learning.
This Cisco 300-620 question asks which bridge domain setting lets a Cisco ACI leaf switch learn the source IP address of a packet entering a front-panel port. The answer is Unicast Routing (D), because enabling it on the BD activates data-plane IP address learning.
The most common wrong choice is L2 Unknown Unicast - Hardware proxy, because candidates associate leaf learning with L2 forwarding behavior instead of the BD's Unicast Routing IP-learning function.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
Enabling Unicast Routing on the ACI bridge domain turns on IP address learning for the leaf switch. When a packet enters a front-panel port, the leaf can learn the source IP address from the data plane if this setting is enabled; without it, the leaf does not perform that IP learning. The question's action — learning a source IP address from a front-panel ingress packet — maps directly to this bridge domain setting. Community replies agree: Odad asked "shouldn't this be unicast routing?" and ed27 confirmed that "unicast routing must be enabled on BD." This is not about routing protocols; it is a BD-level switch that permits the leaf to learn IP information.Why the Other Options Are Wrong
ARP Flooding controls whether ARP requests are flooded within the bridge domain, but it does not make the leaf learn a packet's source IP address. L3 Unknown Multicast Flooding - Flood decides how unknown multicast destinations are handled, not source IP learning. L2 Unknown Unicast - Hardware proxy changes how the leaf handles unknown destination MAC addresses by using the spine proxy, not how it records source IP information. None of these options enables data-plane IP address learning on the ACI leaf.Community Comment Notes
prospio971 explained that with unicast routing enabled on the BD, the leaf can perform IP address learning from the data plane and learn both MAC and IP addresses. Odad's short "shouldn't this be unicast routing?" captures the same conclusion. ed27 also agreed, noting that "unicast routing must be enabled on BD." The vote distribution was unanimous for D, and the comments provide useful confirmation rather than a competing answer.Official Reference
Exam Strategy
Focus on the phrase "learns the source IP address" — in ACI that is a data-plane IP learning function tied to the bridge domain's Unicast Routing setting. Eliminate choices that describe flood control or L2 unknown-unicast handling, since those answer different bridge domain behaviors.
Frequently Asked Questions
Why does Unicast Routing on the bridge domain enable source IP learning?
Unicast Routing is the ACI BD setting that allows the leaf to learn IP addresses from data-plane traffic on front-panel ports. Without it, the leaf does not perform that IP address learning.
Is ARP Flooding involved in learning the source IP of a front-panel packet?
No. ARP Flooding controls how ARP requests are flooded in the bridge domain, while source IP learning is controlled by the BD's Unicast Routing setting.