As shown in Figure 1-1, R1 and R2 are AR routers, R3 is a NE20E router, and R4 a NE40E router. A loopback interface is established on both R2 and R4 to simulate a terminal network segment connected to the two AR routers. Dynamic routing protocols run on the entire network. Traffic is transmitted along the path of R4-R3-R1-R2 by modifying the cost value. It is required that loopback interfaces of R2 and R4 communicate with each other, and traffic be transmitted along the designed path: R4-R3-R1-R2 or R2-R1-R3-R4.
R4 can use the loopback address as the source address to ping the loopback address of R2. However, during a tracert operation, the tracert packet can only reach R1. R2 does not respond, and the message "* * *" is displayed on R2. R2 can also use the loopback address as the source address to ping the loopback address of R4.
Figure 1-1 Networking diagram