"Double Insurance" for AI Servers: Detailed Explanation of Optical Module Dual-Link Architecture

Dual-link design: the lifeline of AI server clusters

The fatal flaw of single-link architectures in thousand-card GPU clusters - the——

Training disruption cost: single Spine switch failure results in huge hourly losses to the enterprise

Latency-sensitive challenges: AllReduce operations require gradient synchronisation latency

Reliability bottleneck: Traditional tree topology has 7 potential single point of failure links

Lessons learned from blood and tears: A real case of an AI company

In Q3 2024, a manufacturer failed to deploy dual links, resulting in:

switch port failure caused 72 minutes of training interruption

Indirect loss: contractual penalty due to delayed model delivery

The dual-link design is the core solution to this pain point.

2. Panoramic analysis of dual-link leaf-spine architecture

Physical topology diagram (including optical module deployment)

Key component description:

Spine switch: fully interconnected backbone, must support 800G OSFP optical module and ECMP

Leaf switch: each switch is connected to two spines through dual optical modules to avoid single point failure

Server connection: use 200G active optical cable (AOC) to directly connect to Leaf

III. Dual-link core technology principle

1. Homogeneous and heterogeneous link adaptation

Dual links can use "homogeneous links" (two links of the same type, such as both InfiniBand HDR) or "heterogeneous links" (such as one InfiniBand for low-latency communication and one Ethernet for large-capacity data transmission)

2. Dynamic link resource allocation

Seamless switching mechanism: Use "active/standby mode" or "load balancing + dynamic adjustment":

Active/standby mode: In normal conditions, the primary link carries the main traffic, and the standby link only transmits heartbeat packets; in case of failure, the standby link takes over all traffic in microseconds to ensure that data is not lost.

Load balancing mode: Two links work at the same time, and the surviving link automatically takes over all traffic after a failure (the protocol layer needs to support traffic redistribution to avoid congestion).

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