How Does ACI Multi-Pod Forward ARP When Flooding Is Disabled?
A Cisco ACI fabric must send a packet between two pods in a Cisco ACI Multi-Pod topology where ARP flooding is disabled within the bridge domain. How does a Cisco ACI spine switch forward ARP messages from a leaf switch in POD1 to POD2?
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 question tests who performs which function in the ARP gleaning process — the leaf originates ARP optimization/gleaning, while the spine is the device that replicates the Glean message to the remote pod using multicast 239.255.255.240.
In a Cisco ACI Multi-Pod fabric with ARP flooding disabled in the bridge domain, a leaf in POD1 cannot flood the ARP request locally, so the spine relays it as an ARP Glean message to the multicast address 239.255.255.240 to reach POD2. This page confirms option D is the correct answer and explains why ARP optimization and proxy ARP do not describe spine-to-spine forwarding.
Many learners pick option A, assuming ARP optimization covers the inter-pod path; ARP optimization is actually a leaf-local feature that answers ARP for COOP-learned remote endpoints and does not describe how the spine forwards the request between pods.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
With ARP flooding disabled inside the bridge domain, the leaf in POD1 cannot use its local multi-destination tree to flood the ARP request, so the request must be carried to the spine as an ARP Glean message. The spine is the device that forwards the gleaned ARP across the Multi-Pod fabric, replicating it toward the leaf nodes in POD2 using the multicast address 239.255.255.240. This is what lets the remote endpoint in the second pod receive the request, learn the sender, and answer it, preserving connectivity even though flooding is suppressed. Option D names both the mechanism (ARP Glean) and the transport (multicast 239.255.255.240), which is exactly the spine-side behavior the question asks about.Why the Other Options Are Wrong
Option A confuses the leaf's role with the spine's: ARP optimization is applied locally by the leaf, which answers ARP requests on behalf of remote endpoints whose IP-to-MAC bindings it learned through COOP, and it does not explain inter-pod forwarding by the spine. Option B is wrong because dropping the ARP request would break endpoint discovery entirely — the whole point of gleaning is to preserve connectivity when flooding is disabled. Option C cites a multicast group that is not the ACI glean group and describes a proxy ARP reply, which is a host/L3 gateway behavior rather than the fabric's spine replication of a glean message.Community Comment Notes
Commenters converge on D with no dissent. As mms101 explains, quoting the Cisco documentation, "the leaf nodes are not allowed to flood the frame along the local multi-destination tree," which is why ARP gleaning was implemented so the request still reaches a remote endpoint. ed27 draws the cleanest distinction: "The option A is for the leaf, while D is for the spine," reinforcing that the option wording must match the device named in the question. prospio971 adds that the glean message is sent to a specific multicast group address associated with the bridge domain, and all 100 recorded votes landed on D.Official Reference
Exam Strategy
Read the device named in the stem first — the question asks about the spine, so eliminate any option describing a leaf-local behavior such as ARP optimization. Memorize the pairing of ARP gleaning with multicast address 239.255.255.240 for Multi-Pod flooding-disabled bridge domains.
Frequently Asked Questions
Why is ARP optimization (option A) wrong for the spine in this Multi-Pod scenario?
ARP optimization is a leaf function that answers ARP locally using COOP-learned remote IP-to-MAC bindings; it does not describe how a spine forwards an ARP request from POD1 to POD2.
What is multicast 239.255.255.240 used for in Cisco ACI Multi-Pod?
It is the destination address the spine uses to forward ARP Glean messages to leaf switches in remote pods when ARP flooding is disabled in the bridge domain.