not configured manually, the router will assign it to be the IP address of the lowest-numbered loop- back address, if one is configured. If a loopback address is not configured, the router ID will be the highest IP address configured on the router. The only ways to change the router ID are to remove and reinstall the OSPF configuration, or to reboot the router. Be careful, and think ahead when planning your network IP scheme. Specific Routing Protocols When first deploying OSPF, engineers commonly make the mistake of neglecting the priority and router ID when configuring the routers. Left to its own devices, OSPF will usually pick routers that you would not choose as the DR and BDR. A common network design using OSPF is to have a WAN in the core as area zero. Figure 10-12 shows such a network. Notice that all of the areas are designated with the same OSPF process number. Each of the areas borders on area zero, and there are no paths between areas other than via area zero. This is a proper OSPF network design. OSPF 100 Area 1 OSPF 100 OSPF 100 OSPF 100 Area 2 Area 0 Area 3 Figure 10-12. Simple OSPF network Area zero does not have to be fully meshed when using technologies such as frame relay. This is in part because OSPF recognizes the fact that there are different types of networks. OSPF knows that networks supporting broadcasts act differently from networks that are point-to-point and thus have only two active IP addresses. OSPF supports the following network types: Point-to-point A point-to-point network is one with only two nodes on it. A common example is a serial link between routers, such as a point-to-point T1. No DR is chosen a point-to-point network because there are only two routers on the network. This is the default OSPF network type on serial interfaces with PPP or HDLC encapsulation. Chapter 10: Routing Protocols Point-to-multipoint A point-to-multipoint network is a network where one hub router connects to all the other routers, but the other routers only connect to the hub router. Specifi- cally, 