Troubleshooting Static Routes

Followers of the assorted American political scandals of the past 30 or so years will have heard a congressional investigator ask the question, "What did he know and when did he know it?" The same question serves an internetworking investigator well. When troubleshooting routing problems, the first step should almost always be to examine the route table. What does the router know? Does the router know how to reach the destination in question? Is the information in the route table accurate? Knowing how to trace a route is essential to successfully troubleshooting an internetwork.

Case Study: Tracing a Failed Route

Figure 3.13 shows a previously configured internetwork, with each router's associated static routes. A problem has been discovered. Devices on subnet 192.168.1.0/27, connected to Pooh's Ethernet interface, can communicate with devices on subnet 10.1.0.0/16 just fine. However, when a ping is sent from Pooh itself to subnet 10.1.0.0/16, the ping fails (Figure 3.14). This seems strange. If packets being routed by Pooh successfully reach their destinations, why do packets originated by the same router fail?

Figure 3.13. Packets from subnet 192.168.1.0/27 to subnet 10.1.0.0/16 are routed correctly, but Pooh itself cannot ping any device on 10.1.0.0/16.

Figure 3.13. Packets from subnet 192.168.1.0/27 to subnet 10.1.0.0/16 are routed correctly, but Pooh itself cannot ping any device on 10.1.0.0/16.

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