Why does BRAS need to be connected to an aggregation switch
They support link aggregation protocols such as Link Aggregation Control Protocol(LACP) and Static Link Aggregation, which allow multiple physical links to be combined into a single logical connection. An Aggregation or "Top-of-Rack" switch is designed to connect everything in a rack at high speeds, then have an even bigger pipe out to the rest of the network. Most home-users do not need this, but enterprise users will see the benefit both from the increased throughput, as well as the redundancy it provides. Even though it is not going to be used for the same purpose, it is common that the connection of dual-attached servers, storage, and other network-based services connect at this level or have their own block of switches that connect to the high-speed core switches in the same manner in the main.
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