ACI Silent Host Detection Mechanism

Answer Correct answer: B — Cisco ACI uses ARP gleaning to detect silent hosts for Layer 3 routed traffic.

When Layer 3 routed traffic is destined to a Cisco ACI fabric, which mechanism does ACI use to detect silent hosts?

  1. gratuitous ARP
  2. ARP gleaning Correct Answer
  3. proxy ARP
  4. inverse ARP
aci silent host detection arp gleaning cisco aci aci coop database layer 3 routed traffic aci

Community Votes

B
100%

100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

The question tests the specific mechanism for silent host detection in ACI; the common trap is confusing it with proxy ARP or gratuitous ARP.

Cisco ACI uses ARP gleaning to detect silent hosts when Layer 3 routed traffic enters the fabric. This page establishes that ARP gleaning triggers the spines to generate an ARP request for destination IPs missing from the COOP database.

Choosing proxy ARP, which allows a router to answer ARP requests on behalf of another device, rather than gleaning to discover silent hosts.

Community Discussion (3 comments)

prospio971 👍 1 Selected: B
In Cisco ACI, ARP gleaning is the mechanism used to detect silent hosts when Layer 3 routed traffic is destined for the fabric. A silent host is an endpoint that has not yet sent any traffic, so its IP-to-MAC mapping is unknown to the ACI fabric (i.e., it is not in the COOP database).
mdriraa 👍 1 Selected: B
https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739989.html
ed27 👍 2 Selected: B
ARP Gleaning, also known as Silent Host Detection, is a process where ACI Spines generate an ARP request for a destination IP address that does not exist in the COOP (Cluster of On-Premises) database

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Expert Analysis

Why the Answer Is Correct

ARP gleaning is the correct mechanism because when L3 routed traffic arrives for an IP not in the COOP database, the ACI spines generate an ARP request to discover the silent host's MAC address. This process is explicitly known as Silent Host Detection in the ACI architecture.

Why the Other Options Are Wrong

Gratuitous ARP is used by hosts to announce their IP-to-MAC mapping proactively, not by the fabric to discover unknown hosts. Proxy ARP allows a router to answer ARP requests on behalf of another host, which is different from gleaning. Inverse ARP is used in Frame Relay to dynamically map DLCIs to IP addresses, which is unrelated to ACI silent host detection.

Community Comment Notes

Users correctly identified ARP gleaning as the mechanism for Silent Host Detection. As ed27 noted, "ARP Gleaning, also known as Silent Host Detection, is a process where ACI Spines generate an ARP request". Prospio971 further clarified that a silent host is an endpoint whose "IP-to-MAC mapping is unknown to the ACI fabric".

Official Reference

https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739989.html

Exam Strategy

Remember that "silent host detection" in ACI is synonymous with ARP gleaning. When the COOP database lacks an endpoint mapping, the spine initiates an ARP request to glean the MAC address.

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