A topology is a description of a layout or arrangement. There are two kinds topology perspective for WANs. One perspective is the physical topology, another perspective is the logical topology, they are different but related.
In physical side of WANs, we talked about he physical layout of the network, which describes the physical arrangement of network devices that allow for data to move from a source to a destination network. In contrast, logical WAN topology describe the path a signal takes through the physical topology. There are 3 types of WAN Topology options.
A star topology involves a central location serving as the hub in the design. In a star topology, while a failure to a regional hub or router will not affect the other sites on the WAN, the central hug or router introduces a single point of failure. The central hub also limited the overall performance for the WAN. Due to the importance placed on the central location serving as the hub, redundant routers often introduced to provide site reliability.
The advantage of a star topology is low cost on equipment and administration labor, however, it suffers disadvantage of low reliability. A full mesh topology, on the other hand, has every site’s WAN router connected to every other site on the wide area network. With such high level of redundency, full mesh topologies provide a high degree of dependability and fault tolerance with high equipment cost and high administrative complexity. In full mesh topology, many virtual circuits are required to establish at least one independent link between any 2 sites, which brings up the cost.
When full mesh removes some redundant connections for cost reduction, we get partial mesh topology. Companies can design a cost effective partial mesh topology that balance the fault tolerance, scalability and budget.