Explanation/Reference:
Distribution Layer:
The distribution layer aggregates the data received from the access layer switches before it is transmitted to the core layer for routing to its final destination. The distribution layer con- trols the flow of network traffic using policies and delineates broadcast domains by per- forming routing functions between virtual LANs (VLANs) defined at the access layer.
VLANs allow you to segment the traffic on a switch into separate subnetworks. For exam- ple, in a university you might separate traffic according to faculty, students, and guests.
Distribution layer switches are typically high-performance devices that have high availabili- ty and redundancy to ensure reliability. You will learn more about VLANs, broadcast domains, and inter-VLAN routing later in this book.
Core Layer
The core layer of the hierarchical design is the high-speed backbone of the internetwork.
The core layer is critical for interconnectivity between distribution layer devices, so it is important for the core to be highly available and redundant. The core area can also connect to Internet resources. The core aggregates the traffic from all the distribution layer devices, so it must be capable of forwarding large amounts of data quickly.
Note
In small networks, it is not unusual to implement a collapsed core model, where the distribution layer and core layer are combined into one layer.