ier ecommerce website The servers at each level connect with servers in the adjacent level or levels. For example, application servers have interfaces in the database layer and the Internet layer. All serv- ers must have multiple interfaces. In the case of high-availability networks, the servers may need four interfaces: two for the level above (one in each switch) and two for the level below. There are two generally accepted methods for accomplishing this layering. I'll call them bridging and routing. In the bridging method, the lower interfaces of the upper layer are connected to the same VLAN as the upper interfaces of the layer beneath them. Figure 35-12 shows an example of a bridged ecommerce design. The advantages of this design include simplicity and speed. There are only three VLANs, and there are no routers or firewalls separating the layers. The disadvantage is decreased security, as there are no devices separating the servers on one layer from the servers on the next. The servers in a bridged design need to talk to the networks directly connected to them. The web servers are the only servers that require a default gateway in this design. The more secure alternative to the bridged design is the routed design. Routing between the layers allows firewalls to be placed between them. Figure 35-13 shows a typical routed three-tier ecommerce design. Chapter 35: Designing Networks Internet Internet VLAN Internet layer Web/App VLAN Application layer App/DB VLAN Database layer Figure 35-12. Bridged three-tier ecommerce design Internet Internet\ VLAN Internet layer Application layer Database layer Figure 35-13. Routed three-tier ecommerce design The routed design is more complicated because there are almost twice as many VLANs, each of which must have an SVI or router interface associated with it. Because there are physical or logical routers separating the VLANs, the servers on each level must have default gateways or route statements configured. The benefit of a routed design is security. Because all traffic mus