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

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CCNA 2 Module 9 Basic Router Troubleshooting

Objectives

• Examining the routing table

• Network testing

• Troubleshooting router issues

The show ip route Command

The show ip route command displays the contents of the IP routing table.

Static Routing

Dynamic Routing

Default Route

Default RouteContinued

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 0.0.0.0/0 is another way to configure a default route.

Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address | exit-interface]

Determining Route Source and Destination

Determining L2 and L3 Addresses

Determining Administrative Distance

Determining the Route Metric

Routing protocols use metrics to determine the best route to a destination.

Determining the Route MetricContinued

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

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.

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

The show ip route network Command

The show ip protocols Command

The show ip rip database Command

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.

Introduction to Network Testing

Structured Approach to Troubleshooting

• 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

• Improperly configured serial interfaces

• Improperly configured Ethernet interfaces

• Improper encapsulation set • Improper clock rate settings

on serial interfaces • Network interface card (NIC)

problems

• Routing protocol not enabled

• Wrong routing protocol enabled

• Incorrect IP addresses

• Incorrect subnet masks

Layer 1 Problems in a Network

Layer 7 Troubleshooting Using Telnet

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.

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.

• 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

The show interfaces serial command

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.

• 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

• 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

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.

Troubleshooting Using show cdp neighbors detail Command

Troubleshooting Layer 3 Using Ping

Troubleshooting Layer 3Using 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.

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.

Troubleshooting Routing Issues

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.

Troubleshooting Routing Issues

The show ip protocols Command

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.

Introduction to debug

Debug syntax

Summary

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

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