The back-to-back link aggregation topology involves two separate systems that are cabled directly to each other, as shown in the following figure. The systems run parallel aggregations.
In this figure, device bge0 on System A is directly linked to bge0 on System B, and so on. In this way, Systems A and B can support redundancy and high availability, as well as high-speed communications between both systems. Each system also has interface ce0 configured for traffic flow within the local network.
The most common application for back-to-back link aggregations is mirrored database servers. Both servers need to be updated together and therefore require significant bandwidth, high-speed traffic flow, and reliability. The most common use of back-to-back link aggregations is in data centers.