Where is the full-mesh topology commonly seen in the three-tier design model?

Distribution layer switches are fully meshed for redundancy. The number of links can be calculated with the formula of N(N – 1). So if you had four distribution switches, the ports required for a full mesh would be 4(4 – 1) = 4 × 3 = 12 ports among the four switches. The formula of N(N – 1) / 2 would give you the number of links (connected ports): 4(4 – 1) / 2 = 4 × 3 / 2 = 6 links. The core layer is normally implemented with a star topology. The access layer is normally implemented with a partial mesh topology or hybrid topology. Routing layer is not a valid term in the Cisco three-tier design model.