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Though BGP supports the traditional Flow-based Layer 3 Equal Cost Multi-Pathing (ECMP) traffic load balancing method, it is not the best fit for a RoCEv2-based AI backend network. This is because GPU-to-GPU communication creates massive elephant flows, which RDMA-capable NICs transmit at line rate. These flows can easily cause congestion in the backend network.
In ECMP, all packets of a single flow follow the same path. If that path becomes congested, ECMP does not adapt or reroute traffic. This leads to uneven bandwidth usage across the network. Some links become overloaded, while others remain idle. In AI workloads, where multiple high-bandwidth flows occur at the same time, this imbalance can degrade performance.
Deep learning models rely heavily on collective operations like all-reduce, all-gather, and broadcast. These generate dense traffic patterns between GPUs, often at terabit-per-second speeds. If these flows are not evenly distributed, a single congested path can slow down the entire training job.
This chapter introduces two alternative load balancing methods to traditional Flow-Based with Layer 3 ECMP: 1) Flowlet-Based Load Balancing with Adaptive Routing, and 2) Packet-Based Load Balancing with Packet Spraying. Both aim to improve traffic distribution in RoCEv2-based AI backend networks, where conventional flow-based routing often Continue reading