ACI Multi-Site ISN Dependencies: MTU and Routed Subinterfaces
Short answer
B The Inter-Site Network requires an MTU of 1600 bytes and utilizes routed subinterfaces on the ISN to support ACI Multi-Site deployment.
Question
Key Takeaways
The main trap is confusing the high MTU (9000 bytes) used for intra-fabric links with the lower, specific MTU (1600 bytes) required for the Inter-Site Network underlay.
This question addresses the specific technical requirements for the Inter-Site Network (ISN) in a Cisco ACI Multi-Site deployment. The correct answer highlights the mandatory MTU of 1600 bytes and the use of routed subinterfaces.
Common mistake: Many candidates select Option C because they associate 'jumbo frames' with ACI fabric performance. They mistakenly apply the 9000-byte MTU rule from internal fabric links to the ISN, which only requires 1600 bytes. They may also incorrectly assume DHCP or strict low-latency multicast are core dependencies.
Expert Analysis
The Inter-Site Network (ISN) in Cisco ACI Multi-Site architecture serves as the underlay transport connecting different sites. Its primary function is to carry overlay traffic, specifically VXLAN encapsulated packets between remote VTEPs.
First, consider the MTU requirements. Because ACI uses VXLAN (26-byte header), Geneve, and potentially other overheads on top of standard Ethernet frames, the underlying network must support a larger Maximum Transmission Unit (MTU) to prevent fragmentation and ensure efficient packet delivery. The official requirement for the ISN is an MTU of 1600 bytes (sometimes cited as requiring jumbo frames up to that size depending on specific configuration, but 1600 is the strict baseline for the ISN subinterfaces). Option B correctly identifies this MTU value.
Second, consider the network type. The ISN consists of routed connections between the Border Leaf switches at each site. These connections are typically implemented using routed subinterfaces (or physical interfaces configured as routed ports) on the Border Leafs. This allows the sites to exchange routing information (like MP-BGP routes) over Layer 3 boundaries. Option B also correctly identifies the use of routed subinterfaces.
Let's evaluate why the other options are incorrect:
- Option A: While multicast can be used for certain control plane functions in some deployments, it is not a hard dependency for the basic ISN connectivity in Multi-Site; unicast MP-BGP is the standard method for route exchange. Furthermore, the latency requirement for general inter-site communication is often higher than 50ms for data traffic, though low latency is preferred for management.
- Option C: An MTU of 9000 bytes is typical for the internal Fabric Links (FLIs) between End Point Modules (EPMs) and Spines/Border Leafs within a single site to accommodate large VXLAN headers plus payload without fragmentation. However, the ISN usually does not require 9000-byte MTUs; 1600 bytes is the sufficient and required minimum. DHCP relay is not a fundamental dependency of the ISN itself.
- Option D: While MP-BGP peering happens over the ISN, the latency requirement for MP-BGP route exchange is generally less stringent than 150 msec for all traffic types, and more importantly, the ISN itself is defined by its Layer 3 connectivity characteristics (MTU and interface type) rather than just the peering protocol latency. The most critical physical/link-layer dependencies are MTU and interface configuration.
Therefore, the two key dependencies are the specific MTU setting and the routed nature of the links.
Exam Strategy
When studying ACI Multi-Site, always distinguish between internal fabric link parameters and external underlay parameters. Memorize that the ISN requires an MTU of 1600 bytes, not 9000 bytes. Also, remember that the ISN connects Border Leafs via Layer 3 routed links (subinterfaces), enabling MP-BGP peering. If you see '9000 bytes' in an option regarding ISN, it is likely a distractor based on internal fabric specs.
Frequently Asked Questions
Why does the ISN require an MTU of 1600 bytes?
Can the ISN use Layer 2 connectivity instead of routed subinterfaces?
Community Votes
100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 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 1
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ostych 👍 1 Answered BISN MTU of 1600 bytes, and routed subinterfaces on the ISN. ISN does not need multicast (A), DHCP (C), nor does it take part in MP-BGP (D)
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