O'Reilly logo
  • Melissa StLaurent thinks this is interesting:

Using designs that connect a larger number of access switches to a small number of distribution switches reduces cabling needs while still allowing all devices to send data to all other devices in the LAN. For example, in Figure 6-12, the design could use a pair of links from each access switch to each of the two distribution switches, providing a great deal of redundancy. With four links for each of 40 access switches, the design uses 160 uplinks. If the design instead did not use distribution switches, to connect a single link between each pair of access switches would require 780 links.


Cover of Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide


I do not understand where the 780 comes from.