Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
In a switched hierarchical design, only the access layer of the enterprise campus module uses Layer 2 switching exclusively. The access layer of the enterprise campus module provides end users with physical access to the network. In addition to using Virtual Switching System (VSS) in place of First Hop Redundancy Protocols (FHRPs) for redundancy, a Layer 2 switching design requires that inter-VLAN traffic be routed in the distribution layer of the hierarchy. Also, Spanning Tree Protocol (STP) in the access layer will prevent more than one connection between an access layer switch and the distribution layer from becoming active at a given time.
In a Layer 3 switching design, the distribution and campus core layers of the enterprise campus module use Layer 3 switching exclusively. Thus a Layer 3 switching design relies on FHRPs for high availability. In addition, a Layer 3 switching design typically uses route filtering on links that face the access layer of the design.
The distribution layer of the enterprise campus module provides link aggregation between layers. Because the distribution layer is the intermediary between the access layer and the campus core layer, the distribution layer is the ideal place to enforce security policies, provide load balancing, provide Quality of Service (QoS), and perform tasks that involve packet manipulation, such as routing. In a switched hierarchical design, the switches in the distribution layer use Layer 2 switching on ports connected to the access layer and Layer 3 switching on ports connected to the campus core layer.
The campus core layer of the enterprise campus module provides fast transport services between the modules of the enterprise architecture module, such as the enterprise edge and the intranet data center.
Because the campus core layer acts as the network's backbone, it is essential that every distribution layer device have multiple paths to the campus core layer. Multiple paths between the campus core and distribution layer devices ensure that network connectivity is maintained if a link or device fails in either layer. In a switched hierarchical design, the campus core layer switches use Layer 3 switching exclusively.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 3, Distribution Layer Best Practices, pp. 97-99 Cisco: Cisco SAFE Reference Guide: Enterprise Campus