1 © 2003, Cisco Systems, Inc. All rights reserved. CCNA 2 Module 9 Basic Router Troubleshooting

222 © 2003, Cisco Systems, Inc. All rights reserved. Objectives Examining the routing table Network testing Troubleshooting router issues

333 © 2003, Cisco Systems, Inc. All rights reserved. The show ip route Command The show ip route command displays the contents of the IP routing table.

444 © 2003, Cisco Systems, Inc. All rights reserved. Static Routing

555 © 2003, Cisco Systems, Inc. All rights reserved. Dynamic Routing

666 © 2003, Cisco Systems, Inc. All rights reserved. Default Route

777 © 2003, Cisco Systems, Inc. All rights reserved. Default Route Continued

888 © 2003, Cisco Systems, Inc. All rights reserved. Configuring a Default Route The ip default-network command establishes a default route in networks using dynamic routing protocols. Router(config-router)#ip default-network network-number Creating an ip route to /0 is another way to configure a default route. Router(config)#ip route [next-hop-ip-address | exit-interface]

999 © 2003, Cisco Systems, Inc. All rights reserved. Determining Route Source and Destination

10 © 2003, Cisco Systems, Inc. All rights reserved. Determining L2 and L3 Addresses

11 © 2003, Cisco Systems, Inc. All rights reserved. Determining Administrative Distance

12 © 2003, Cisco Systems, Inc. All rights reserved. Determining the Route Metric Routing protocols use metrics to determine the best route to a destination.

13 © 2003, Cisco Systems, Inc. All rights reserved. Determining the Route Metric Continued Factors such as bandwidth and delay are static because they remain the same for each interface until the router is reconfigured or the network is redesigned. Factors such as load and reliability are dynamic because they are calculated for each interface in real-time by the router

14 © 2003, Cisco Systems, Inc. All rights reserved. Determining the Route Next Hop Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router.

15 © 2003, Cisco Systems, Inc. All rights reserved. Determining the Last Routing Update Use the following commands to find the last routing update: show ip route show ip route network show ip protocols show ip rip database

16 © 2003, Cisco Systems, Inc. All rights reserved. The show ip route Command

17 © 2003, Cisco Systems, Inc. All rights reserved. The show ip route network Command

18 © 2003, Cisco Systems, Inc. All rights reserved. The show ip protocols Command

19 © 2003, Cisco Systems, Inc. All rights reserved. The show ip rip database Command

20 © 2003, Cisco Systems, Inc. All rights reserved. Observing Multiple Paths to a Destination Some routing protocols support multiple paths to the same destination. Unlike single path algorithms, these multi- path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable.

21 © 2003, Cisco Systems, Inc. All rights reserved. Introduction to Network Testing

22 © 2003, Cisco Systems, Inc. All rights reserved. Structured Approach to Troubleshooting

23 © 2003, Cisco Systems, Inc. All rights reserved. Broken cables Disconnected cables Cables connected to the wrong ports Intermittent cable connection Wrong cables used for the task at hand Transceiver problems DCE cable problems DTE cable problems Devices turned off Typical Layer 1 Errors

24 © 2003, Cisco Systems, Inc. All rights reserved. Improperly configured serial interfaces Improperly configured Ethernet interfaces Improper encapsulation set Improper clock rate settings on serial interfaces Network interface card (NIC) problems Typical Layer 2 Errors

25 © 2003, Cisco Systems, Inc. All rights reserved. Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks Typical Layer 3 Errors

26 © 2003, Cisco Systems, Inc. All rights reserved. Layer 1 Problems in a Network

27 © 2003, Cisco Systems, Inc. All rights reserved. Layer 7 Troubleshooting Using Telnet

28 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using link light Most interfaces or NICs have indicator light that show whether there is a valid connection. This light is referred to as the link light. A faulty or incorrect cable could result in a link light indicating a bad connection or no link If the interface has indicator light that do not show a valid connection, power off the device and reset the interface card Check to make sure that all cables are connected to the ports. Make sure that proper cable is being used. Before running diagnostics or attempt trobleshooting, always check to make sure that the device is powered on.

29 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using show interfaces Command The show interfaces serial command The show interfaces serial command output are displayed as the line (Layer 1)and data-link protocol (Layer 2) status.

30 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using show interfaces Command The second parameter (protocol) indicates whether the line protocol consider the interface usable. This is determined by whether keepalives are successfully received. Keepalives are defined as messages sent by one network device to inform another network device that the virtual circuit between the two is still active. If the interface misses three consecutive keepalives, the line protocol is marked as down. If the interface is up and the line protocol is down, a Layer 2 problem exists. Among the possible causes are: No keepalives No clock rate Mismatch in encapsulation type

31 © 2003, Cisco Systems, Inc. All rights reserved. The show interfaces serial command Troubleshooting Layer 1 Using show interfaces Command - Continued

32 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using show interfaces Command - Continued An increasing number of carrier transitions counts on a serial link may indicate one or more of the following problems: Line interruptions due to problems in the service provider network. Faulty switch, DSU, or router hardware.

33 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using show interfaces Command - Continued If an increasing number of input errors appear, there are several possible sources of those errors. Some of these related to Layer 1 problems are: Faulty telephone company equipment Noisy serial line Incorrect cable or cable length Damaged cable or connection Defective CSU or DSU Defective router hardware

34 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 1 Using show interfaces Command - Continued Another area to examine is number of interface resets. These are the result of too many missed keepalives. The following Layer 1 problems could be a cause of interface resets: Bad line causing carrier transitions Possible hardware problem at the CSU, DSU, or switch If carrier transitions and interface resets are increasing or if input errors are high while interface resets are increasing, the problem is likely to be a bad link or defective CSU or DSU

35 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 2 Using show interfaces Command

36 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Using show cdp neighbors Command If the physical layer is properly functioning, then all other directly connected Cisco devices should be displayed. If no known device appears, a Layer 1 problem likely exists.

37 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Using show cdp neighbors detail Command

38 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 3 Using Ping

39 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Layer 3 Using traceroute Command Traceroute also provides information indicating the relative performance of links: the round trip time (RTT) is the time required to send an echo packet and get a response. If an asterisk (*) appears, the packet failed.

40 © 2003, Cisco Systems, Inc. All rights reserved. The traceroute Operation The followings are the traceroute operation: Traceroute sends out a sequence of User Datagram Protocol (UDP) datagrams from the router to an invalid port address on the remote host. For the first sequence of three datagrams sent, a Time-To-Live (TTL) field value is set to one. The TTL value of 1 causes the datagram to time out at the first router in the path. This router then responds with an ICMP Time Exceeded Message (TEM) indicating that the datagram has expired. Three more UDP messages are now sent, this time with the TTL value set to 2. This causes the second router to return ICMP TEMs. This process continues until the packets actually reach the other destination. Since these datagrams are trying to access an invalid port at the destination host, ICMP Port Unreachable Messages are returned instead of the ICMP Time Exceeded Message. This indicates an unreachable port and signals the traceroute program, terminating the process.

41 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Routing Issues The show ip route Command If the output of the show ip route command does not show the expected learned routes or no learned routes, then the problem is possibly that routing information is not being exchanged. In this case, use the show ip protocols command on the router to check for a routing protocol configuration error.

42 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Routing Issues The show ip protocols Command

43 © 2003, Cisco Systems, Inc. All rights reserved. Troubleshooting Using show controllers serial Command The show controllers serial Command By examining the show controllers serial command output, the type of cable that the controller detects can be determined. This is useful for finding a serial interface with no cable, the wrong type of cable, or a defective cable.

44 © 2003, Cisco Systems, Inc. All rights reserved. Introduction to debug Debug syntax

45 © 2003, Cisco Systems, Inc. All rights reserved. Summary