networking
AI infra interview questions tagged networking, across every topic.
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Concepts behind "networking"
The curriculum that explains the ideas these questions test.
Foundational
Scale-Out Fabric Choice: InfiniBand XDR vs Spectrum-XOutside the NVLink domain every GPU talks over a scale-out fabric, and as of September 2026 NVIDIA sells two at the same 800 Gb/s per port: Quantum-X800 InfiniBand and Spectrum-X Ethernet. They differ in congestion handling, operational familiarity and what happens when something misbehaves rather than in headline speed. The switch radix decides how many endpoints a two-tier fabric reaches, and that single number drives the switch count, the cable count and a large part of the budget.🖧 Hardware & Cluster Build-Out
Foundational
Debugging a Slow All-ReduceA training job reports its all-reduce at a third of what the fabric should deliver, every node passed its health check, and nothing is logged. This page is the isolation order that finds the cause in an hour instead of a day: measure the collective in isolation, split the job until the slow pair or rank appears, then check the specific things that make a link, a node or a placement slow. Most cases end at one NIC, one topology mismatch, or GPUDirect silently off.🔌 Networking & Storage
Core
RDMA, InfiniBand and RoCEv2Training across nodes moves hundreds of gigabytes per step, and a CPU-driven TCP stack cannot feed a 400 Gb/s link. RDMA lets a NIC write straight into a remote GPU's memory with no kernel and no copies, and it runs over two fabrics: InfiniBand, which is lossless by design, and RoCEv2, which is Ethernet made lossless by configuration. The choice is operational as much as technical, and the numbers that decide it are per-GPU bandwidth, the collective's volume, and who will debug a pause storm at 3 a.m.🔌 Networking & StorageSign in
Advanced
Rail-Optimized and Fat-Tree FabricsA GPU cluster's network is built from two ideas: a fat tree (Clos) that gives every node a path to every other node with a chosen amount of oversubscription, and rail optimization, which wires GPU i of every node to the same leaf switch so the collectives that dominate training stay one hop away. Sizing one is arithmetic on port counts, and the interview question is usually that arithmetic: how many switches, what oversubscription, and where the NVLink domain ends and the fabric begins.🔌 Networking & Storage🔒 Premium
Advanced
Congestion Control for AI FabricsCollective traffic is the worst case a network can see: hundreds of senders transmit to the same receiver at the same instant (incast), every flow is large and long-lived, and RDMA cannot tolerate a dropped packet. Congestion control is the set of mechanisms (PFC, ECN with DCQCN, adaptive routing, packet spraying) that keep queues from overflowing without stalling the fabric. On plain Ethernet a busy all-reduce can fall to about 60% of link rate; with a tuned control loop it holds above 90%. Reading the counters that show which one you have is the on-call skill.🔌 Networking & Storage🔒 Premium
Advanced
GPUDirect RDMA and GPUDirect StorageBy default a byte leaving a GPU for the network or the disk makes a detour through host memory, crossing PCIe twice and costing a CPU copy. GPUDirect RDMA lets the NIC read and write GPU memory directly, and GPUDirect Storage does the same for NVMe. The win is not raw bandwidth (PCIe is the ceiling either way) but the halving of PCIe traffic and the removal of the host as a bottleneck, which is what makes collectives run at NIC rate and checkpoints run at drive rate. When it is silently off, everything still works, at half speed.🔌 Networking & Storage🔒 Premium