Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
A small branch office typically uses a single Integrated Services Router (ISR), combines LAN and WAN termination, and does not include a distribution layer. Cisco defines a small branch office as an office that contains up to 50 users and that implements a one-tier design. A single-tier design combines LAN and WAN termination into a single ISR, where a redundant link to the access layer can be created if the ISR uses an EtherChannel topology versus a trunked topology, which offers no link redundancy. Because a small branch office uses a single ISR, such as the ISR G2, to provide LAN and WAN services, an external access switch, such as the Cisco 2960, is not necessary. In addition, Rapid PerVLAN Spanning Tree Plus (RPVST+) is not supported on most ISR platforms.
Medium and large branch offices typically use RPVST+ and external access switches. RPVST+ is an advanced spanning tree algorithm that can prevent loops on a switch that handles multiple virtual LANs (VLANs). RPVST+ is typically supported only on external switches and advanced routing platforms.
External access switches provide high-density LAN connectivity to individual hosts and typically aggregate links on distribution layer switches.
Cisco defines a medium branch office as an office that contains between 50 and 100 users and that implements a two-tier design. A dual-tier design separates LAN and WAN termination into multiple devices.
A medium branch office typically uses two ISRs, with one ISR serving as a connection to the headquarters location and the second serving as a connection to the Internet. In addition, the two ISRs are typically connected by at least one external switch that also serves as an access layer switch for the branch users.
Cisco defines a large branch office as an office that contains between 100 and 200 users and that implements a three-tier design. Similar to a dual-tier design, a triple-tier design separates LAN and WAN termination into multiple devices. However, a triple-tier design separates additional services, such as firewall functionality and intrusion detection. A large branch office typically uses at least one dedicated device for each network service. Whereas small and medium branch offices consist of only an edge layer and an access layer, the large branch office also includes a distribution layer.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 7, Enterprise Branch Profiles, pp. 275-279 Cisco: LAN Baseline Architecture Branch Office Network Reference Design Guide: Small Office Design (PDF)
Cisco: LAN Baseline Architecture Branch Office Network Reference Design Guide: Branch LAN Design Options (PDF)