ACI Endpoint Communication within Same EPG
Short answer
A Enable Unicast Routing on the Bridge Domain to allow Layer 3 routing between endpoints in the same EPG connected to different leaf switches.
Question
Key Takeaways
The core concept tested is ACI's default behavior for intra-EPG traffic, specifically the requirement of Unicast Routing for inter-leaf communication.
This question addresses how to resolve Layer 2 connectivity issues between endpoints in the same EPG but different Bridge Domains in Cisco ACI. It establishes that enabling Unicast Routing on the Bridge Domain is the correct configuration.
Common mistake: Many learners incorrectly select 'Configure a contract' (B) because they overgeneralize ACI's strict policy model, failing to realize contracts are not needed for same-EPG traffic.
Expert Analysis
Why the Answer Is Correct
In Cisco ACI, endpoints in the same EPG communicate via flooding or hardware proxying by default. However, when endpoints are attached to different leaf switches (Leaf-101 and Leaf-102), direct Layer 2 forwarding may fail if the fabric relies on routing or specific proxy settings. Enabling 'Unicast Routing' on the Bridge Domain allows the fabric to route traffic between the endpoints using the BD's subnet, ensuring connectivity across the spine.
Why the Other Options Are Wrong
Option B is incorrect because contracts enforce communication between different EPGs; same-EPG traffic is allowed by default. Option A ('Hardware Proxy') is typically used for ARP/ND optimization, not primary L2/L3 reachability. Option D ('Silent Host Detection') helps with DHCP learning but does not solve basic IP reachability.
Community Comment Notes
Commenters like prospio971 correctly identify that 'Unicast Routing' is the key missing feature, as noted in their explanation about Layer 3 routing between endpoints. Another user mentions silent host detection, which is less relevant than the routing configuration for this specific failure mode.
Exam Strategy
When troubleshooting ACI connectivity, always check if the Bridge Domain has Unicast Routing enabled if endpoints cannot reach each other across leaves. Remember that same-EPG traffic does not require contracts.
Frequently Asked Questions
Why isn't a contract needed for same-EPG communication?
Does Unicast Routing affect Layer 2 traffic?
Community Votes
50% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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prospio971 👍 2 Answered ATo allow Endpoint-1 to communicate with Endpoint-2, ensure that the bridge domain (BD-A) has "Unicast Routing" enabled. This feature allows Layer 3 routing between endpoints in the same bridge domain but connected to different leaf switches.
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prospio971 👍 1 Answered BEPG Communication in Cisco ACI: In Cisco ACI, communication between endpoints in the same or different Endpoint Groups (EPGs) requires a contract. By default, no traffic is allowed between EPGs or even within the same EPG unless explicitly permitted by a contract. Why a Contract is Needed? Even though both endpoints are in the same EPG (EPG-A), Cisco ACI enforces policy-driven communication. Without a contract, traffic between Endpoint-1 and Endpoint-2 will be dropped.
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samfab 👍 1 Answered DEnable silent host detection
Reader comments & corrections
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