Traditional data centers were built using a three-tier architecture with core, aggregation, and access layers, or a two-tier collapsed core with the aggregation and core layers combined into one layer, as shown in the following figure. This architecture provides optimal packet forwarding when the data flow follows a north-south traffic pattern, where one end of the traffic flow is outside the DC (the WAN link or the campus network) and the other end is the servers hosted within the DC. This was the common type of traffic in client server applications:

Figure 1: DC tiered architecture

As we have seen in the earlier chapters, traffic ...

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