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CCNA Exploration: Routing Protocols and Concepts  Chapter 1 Case Study
Objectives: • • Describe the structure of a routing table. Describe how a router determines a path and switches packets.

Intro: After receive a call from OCP Enterprises, you place a visit to their office to check the problem. According to the complaint made during the phone call, parts of their network are not able toreach each other. The reported problems are listed below: 1. 2. 3. PC1 is not able to reach PC4 with no packets being successfully sent/received between those PCs. PC1 is not able to reach PC5, PC6 or PC7. PC1 and PC3 reach network 3 but PC2 does not.

Topology:

© 2009 Cisco Learning Institute

   

CCNA Exploration: Routing Protocols and Concepts  Chapter 1 Case Study
The Scenario: Thetopology is relatively simple and you decide to examine each problem separately.

Problem 1 – PC1 fails reaching PC4 You take a look on the topology handed to you (shown above) and, from PC1, you send a few packets to PC4 using the ping command. You watch while all packets are being dropped. You check PC1’s IP information configuration and it looks good: PC1 has a valid IP address belonging toR1’s network 1 and PC1’s default gateway is the R1’s interface connected to network 1. In order to identify the failure point, you decide to issues pings to closer devices. You issue another ping from PC1 but this time targeting R1’s fa0/0 and watch them being successfully received by R1. The successful ping from PC1 to R1 fa0/0 (PC1’s default gateway) means the path from PC1 to R1 is working fine (ata OSI Layer 3, at least) Based on the topology shown above, you move forward and ping R2’s interface Se0/0 from PC1. All pings fail. Looks like R2 has wrong routing information regarding network 1 but you still need a few more tests to make sure.

Question 1: Could you give one reason why is it too early to blame R2’s routing table? Answer: At this point there is no guarantee that the seriallink between R1 and R2 is up. A problem in R1’s side of the serial link could bring the link down, keep PC1 from reaching PC4 and R2 would have no blame on it. You issue a ping from R1 to R2’s serial 0/0. Since the pings were successful, you assume R2 must have some wrong information regarding network 1 placed on its routing table.

Question 2: How the successful ping from R1 to R2 raised the oddsof problems in R2’s routing table? Answer: The successful pings from R1 to R2 serial 0/0 ensure that link is fine. Network 1 is direct connected to R1 and therefore, R1 must know how to reach network 1. Because the entire path between PC1 and R2 is ok at layer 1 and 2 levels, on this case, chances are R2 routing information is the failure point.

© 2009 Cisco Learning Institute

   CCNA Exploration: Routing Protocols and Concepts  Chapter 1 Case Study
As a last test, you issue a ping from R2 to PC1 and watch them fail. A quick look at R2’s routing table reveals the problem: Routing Information Known by R2: Destination Network 192.168.4.0/24 192.168.3.0/24 192.168.5.0/24 Address of the Next Hop [directly connected] [directly connected] [directly connected]

Question 3: Whichproblem on R2’s routing table keeps PC1 from reaching PC4? Answer: R2 has no route to reach network 1. In other words, R2 does not know how to send packets to network 1.

Question 4: (Challenge Question) If a ping from R1 to PC4 was issued would it be successful? Explain. Answer: Yes, it would. R2 doesn’t know how to reach network 1 (192.168.1.0/24) but it knows how to reach R1 because R1 lays inone of R2’s direct connected networks (192.168.5.0/24). R2’s routing information showed above has network 5 included on it. You notice the problem in R2 and manually add a static route to network 1 using R1’s interface serial0/0 IP address (192.168.5.1) as the next hop. R2’s updated routing information looks like the table shown below: R2’s Manually Updated Routing Information Destination...