ers 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 33-13 shows an example of a bridged e-commerce design. The advantages of Chapter 33: Designing Networks 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. Internet Less secure Internet layer Web servers Application layer Application servers Database layer Database servers Most secure Figure 33-12. Typical three-tier e-commerce web site Internet Internet VLAN Internet layer Web/App VLAN Application layer App/DB VLAN Database layer Figure 33-13. Bridged three-tier e-commerce design The servers in a bridged design only 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. Network Designs The more secure alternative to the bridged design is the routed design. Routing between the layers allows firewalls to be placed between the layers. Figure 33-14 shows a typical routed three-tier e-commerce design. Internet Internet VLAN Internet layer Application layer Database layer Figure 33-14. Routed three-tier e-commerce 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 must pass through a router, the router can be a firewall. The problem with both of