ltiple routing protocols on a single Ethernet network A network may also contain multiple instances of the same routing protocol. These separate areas of control are called autonomous systems in EIGRP, and processes in OSPF (although the term autonomous system is often used incorrectly). Each instance is referenced with a number-either an autonomous system number (ASN), or a process ID (PID). Figure 10-2 shows a network with two OSPF processes active. Because the multicast packets sent by an OSPF router will be destined for All OSPF Routers, all OSPF rout- ers will listen to the updates. The updates contain the PIDs, SO the individual routers can determine whether to retain or discard them. (RIP does not support the idea of separate processes, SO any router running RIP will receive and process updates from all other RIP routers on the network.) When there are two processes on the same network, the routes learned in each are not shared between the processes by default. For routes to be shared, one of the rout- ers must participate in both processes, and be configured to share the routes between them. The act of passing routes from one process or routing protocol to another process or routing protocol is called redistribution. An example of multiple OSPF routing pro- cesses being redistributed is shown in Figure 10-3. Chapter 10: Routing Protocols Router A Router B Router C Router D Router E Figure 10-2. Two OSPF processes on a single network Router A Router B Router C Router D Router E Figure 10-3. Routing protocol redistribution In Figure 10-2, we had two OSPF processes, but there was no way for the processes to learn each other's routes. In Figure 10-3, Router E is configured to be a member of OSPF process 100 and OSPF process 200. Router E thus redistributes routes learned on each process into the other process. When a route is learned within a routing process, the route is said to be internal. When a route is learned outside the routing process, and redistributed into the pro- cess, the route is said to be ext