Round-Robin distribution

All outgoing traffic is spread evenly across all of the adapters in the EtherChannel. It provides the highest bandwidth optimization for the AIX® server system. While round-robin distribution is the ideal way to use all of the links equally, consider that it also introduces the potential for out-of-order packets at the receiving system.

In general, round-robin mode is ideal for back-to-back connections running jumbo frames. In this environment, there is no intervening switch, so there is no chance that processing at the switch could alter the packet delivery time, order, or adapter path. On this direct cable network path, packets are received exactly as sent. Jumbo frames (9000 byte MTU) always yield better file transfer performance than traditional 1500 byte MTUs. In this case, however, they add another benefit. These larger packets take longer to send so it is less likely that the receiving host would be continuously interrupted with out-of-order packets.

Round-robin mode can be implemented in other environments but at increased risk of out-of-order packets at the receiving system. This risk is particularly high when there are few, long-lived, streaming TCP connections. When there are many such connections between a host pair, packets from different connections could be intermingled, thereby decreasing the chance of packets for the same connection arriving out-of-order. Check for out-of-order packet statistics in the tcp section of the netstat -s command output. A steadily-increasing value indicates a potential problem in traffic sent from an EtherChannel.

If out-of-order packets are a problem on a system that must use traditional Ethernet MTUs and must be connected through a switch, try the various hash modes offered in standard mode operation. Each mode has a particular strength, but the default and src_dst_port modes are the logical starting points as they are more widely applicable.