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Consider the following scenario, you have created a LAG interface to increase total bandwidth between 2 network nodes, usually, these are switches. For testing purposes to make sure that the LAG interface is working properly you have attached two servers that transfer data, most commonly the well-known network performance measurement tool https://en.wikipedia.org/wiki/Iperf is used to test such setups. For example, you might have made a LAG interface out of two Gigabit Ethernet ports, which gives you a 2Gbps interface while virtual interface that can load balance traffic over both interfaces and theoretically reach 2Gbps throughput, while the servers are connected using a 10Gbps interface, for example, SFP+.
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