CHAPTER Routers 7 NETWORK LAYER As discussed in the previous chapter, link layer systems can interconnect end systems at the scale of a local area network. However, scaling a network built of bridges and switches to global scale is not feasible. The filtering database would be very large, broadcast storms would limit the operation efficiency, and routing would be ineffi- cient due to the spanning tree algorithm. Therefore, it is necessary to use systems that are specifically designed to achieve global connectivity. These network layers systems (or "routers") overcome the limitations of link layer systems. Routers interconnect local area networks, and the resulting network of networks is an Internet that spans the globe. Due to the size of the resulting network, it is important to consider scalability as one of the key aspects of router design.