Detecting Silent Endpoints in Cisco ACI L3BD
A Cisco ACI fabric must detect all silent endpoints for the Layer 3 bridge domain. Which actions accomplish this goal?
Community Votes
67% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The exam tests knowledge of how Layer 2 vs. Layer 3 domains handle endpoint learning. The common trap is confusing 'Unicast Routing' with 'Multicast' or assuming that disabling routing helps discovery, whereas only enabled routing facilitates the necessary ARP-based detection.
This question addresses the specific configuration required to detect silent endpoints within a Layer 3 Bridge Domain (L3BD) in a Cisco ACI fabric. The correct action is to enable unicast routing, which allows the system to use ARP requests for endpoint discovery.
Many candidates select 'Disable Unicast Routing' (Option B/D), mistakenly believing that keeping the domain purely Layer 2 prevents unnecessary traffic or that silent devices do not need routing. This is incorrect because without unicast routing, the fabric cannot effectively probe for these endpoints using standard IP mechanisms.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
Enabling Unicast Routing on a Layer 3 Bridge Domain (L3BD) is the definitive method for detecting silent endpoints. When unicast routing is enabled, the fabric treats the BD as a routed subnet. This allows the Border Leaf switches to send gratuitous ARP (GARP) or ARP requests into the segment to discover endpoints that are not actively sending traffic (silent devices like printers or IoT sensors). Without this feature, the fabric relies solely on active traffic flows, leaving silent endpoints undetected.Why the Other Options Are Wrong
Disabling Unicast Routing (Options B and D) reverts the Bridge Domain to a pure Layer 2 mode. In this state, the fabric does not perform Layer 3 endpoint resolution via ARP probes, making it impossible to reliably detect silent devices. Option C is identical to Option A in text ('Enable Unicast Routing'), but since Option A is the primary selection supported by the majority of expert consensus and the logical flow of the exam bank, A is the designated correct choice. Note: The options provided in the prompt contain duplicates (A/C and B/D are identical), which is likely an error in the source material, but A is the accepted answer key.Community Comment Notes
Community members overwhelmingly support Option A. As one commenter noted, enabling unicast routing allows 'endpoint discovery using ARP requests.' Another user simply confirmed 'Should be A,' reinforcing that this is a known exam objective. The confusion often stems from the duplicate options, but the core concept remains that routing must be enabled for L3BD silent endpoint detection.Exam Strategy
When dealing with Cisco ACI Bridge Domains, always remember: Layer 2 BDs learn endpoints via MAC address tables from traffic; Layer 3 BDs learn endpoints via ARP/Routing tables. If the question asks about discovering 'silent' or 'passive' endpoints in an L3 context, look for configurations that enable Layer 3 resolution (Unicast Routing).
Frequently Asked Questions
Why can't silent endpoints be detected in a Layer 2 BD?
In a pure Layer 2 BD, endpoints are learned only when they send traffic. Silent devices never initiate traffic, so they remain invisible to the MAC address table.
Does enabling unicast routing cause extra network traffic?
Yes, it generates ARP requests to probe for endpoints. However, this is a controlled overhead necessary for accurate inventory and policy application in ACI.