Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
The distribution layer provides load balancing and redundant paths to the default gateway. The hierarchical model divides the network into three distinct components:
Core layer

Distribution layer

Access layer

The core layer of the hierarchical model provides fast convergence. The core layer typically provides the fastest switching path in the network. As the network backbone, the core layer is primarily associated with low latency and high reliability. The functionality of the core layer can be collapsed into the distribution layer if the distribution layer infrastructure is sufficient to meet the design requirements. Thus the core layer does not contain physically connected hosts. For example, in a small enterprise campus implementation, a distinct core layer may not be required, because the network services normally provided by the core layer are provided by a collapsed core layer instead.
The distribution layer serves as an aggregation point for access layer network links. Because the distribution layer is the intermediary between the access layer and the core layer, the distribution layer is the ideal place to enforce security policies, to provide Quality of Service (QoS), and to perform tasks that involve packet manipulation, such as routing. Summarization and next-hop redundancy are also performed in the distribution layer.
The access layer provides Network Admission Control (NAC). NAC is a Cisco feature that prevents hosts from accessing the network if they do not comply with organizational requirements, such as having an updated antivirus definition file. NAC Profiler automates NAC by automatically discovering and inventorying devices attached to the LAN. The access layer serves as a media termination point for endpoints, such as servers and hosts. Because access layer devices provide access to the network, the access layer is the ideal place to perform user authentication.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 2, Distribution Layer, pp. 43-44 Cisco: Campus Network for High Availability Design Guide: Distribution Layer