How Does Cisco ACI Detect a Moved Silent Host's IP Address?
How does Cisco ACI detect the IP address of a silent host that moved from one location to another without notifying a Cisco ACI leaf?
Community Votes
83% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question tests silent host detection in ACI: only ARP flooding in the bridge domain forces a non-communicating host to answer from its new leaf, while the classic trap is confusing the bounce entry (a consequence of a detected move) with the detection mechanism itself.
Cisco ACI detects a silent host that moved without notifying a leaf by flooding ARP requests inside the bridge domain, because the flooded request reaches the host at its new location. This page confirms option C and explains why bounce entries and COOP announces only follow a move that has already been learned.
Picking D — bounce entries are installed on the original leaf after the move has already been detected through COOP; they redirect traffic toward the new location but never probe for or discover a silent host that has not yet sent any traffic.
Community Discussion (4 comments)
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Expert Analysis
Why the Answer Is Correct
A silent host sends no traffic and no gratuitous ARP, so after it physically moves the original leaf keeps a stale endpoint entry pointing at the old port. When ARP flooding is enabled on the bridge domain, an ARP request for that IP is flooded across the BD instead of being proxy-answered locally from the stale database entry, so the frame reaches the host on its new leaf. The host replies, the new leaf learns the endpoint, and the move is signalled through COOP with a newer sequence number so the old leaf withdraws its entry. Without ARP flooding the request never leaves the old leaf's local proxy reply, and the move stays invisible until the silent host itself starts sending traffic. That is precisely why option C is the correct detection method.Why the Other Options Are Wrong
Option A is a truism rather than a mechanism — the fabric does not spontaneously detect anything; it needs a configured feature such as ARP flooding to reach the moved host. Option B describes what happens after an endpoint is already learned: the leaf announces the endpoint to COOP so other leafs can update their databases, which is the result of detection, not the way a silent host is found. Option D is the most tempting distractor: a bounce entry is installed on the original leaf once a COOP update tells it the endpoint now lives elsewhere, and it forwards traffic to the new leaf, but it cannot discover a location the fabric has never learned. As Rollizo put it, "Bounce doesn't fix the silent hosts."Community Comment Notes
mdriraa explains the benefit of ARP flooding clearly: once a silent IP moves "from one location to another without notifying an ACI leaf," the flooded ARP request reaches the host at its new location and the fabric relearns it. prospio971 reaches the same conclusion, noting that detection "depends on how the bridge domain is configured" and that ARP flooding is the enabler. udo2020 voted for D and cited Cisco's endpoint-learning white paper, but that document describes bounce entries in the context of a move already detected via COOP, which supports C rather than refuting it — the 83-to-17 vote split reflects the same reading."Official Reference
Exam Strategy
On 300-620, separate detection from reaction: ask yourself which option generates traffic that can reach an unknown location, and which option only reacts after the location is known. ARP flooding generates the traffic; COOP announces and bounce entries are reactions, so they can be eliminated quickly.
Frequently Asked Questions
Why isn't a bounce entry the right answer for a moved silent host?
A bounce entry is installed on the old leaf only after the move is detected via COOP, so it redirects traffic to the new location instead of discovering where a silent host went.
What happens if ARP flooding is disabled in the bridge domain?
The leaf proxy-replies from its stale endpoint entry, the ARP request never reaches the moved host, and the new location stays undetected until the silent host sends traffic.