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1© 2004, Cisco Systems, Inc. All rights reserved.
Chapter 8
TCP/IP Suite Error and Control Messages/ Basic Router Troubleshooting
222© 2004, Cisco Systems, Inc. All rights reserved.
Objectives
333© 2004, Cisco Systems, Inc. All rights reserved.
Internet Control Message Protocol (ICMP)
444© 2004, Cisco Systems, Inc. All rights reserved.
Error Reporting and Error Correction
• ICMP is an error-reporting protocol for IP.
555© 2004, Cisco Systems, Inc. All rights reserved.
ICMP Message Delivery
• ICMP messages are encapsulated as data in datagrams in the same way any other data is delivered using IP.
666© 2004, Cisco Systems, Inc. All rights reserved.
Unreachable Networks
777© 2004, Cisco Systems, Inc. All rights reserved.
Ping
888© 2004, Cisco Systems, Inc. All rights reserved.
Detecting Excessively Long Routes
• Whether the actual path includes too many hops or a circular routing path exists, the packet will eventually reach the end of its life, known as its time-to-live (TTL).
999© 2004, Cisco Systems, Inc. All rights reserved.
ICMP Message Types
101010© 2004, Cisco Systems, Inc. All rights reserved.
Destination Unreachable Message
111111© 2004, Cisco Systems, Inc. All rights reserved.
Code Values for Destination Unreachable Messages
121212© 2004, Cisco Systems, Inc. All rights reserved.
TCP/IP Suite Control Messages
• Control messages are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network.
131313© 2004, Cisco Systems, Inc. All rights reserved.
ICMP Redirect/Change Requests
The default gateway of the host may need to use a redirect/change request to inform the host of the best path to a certain network.
141414© 2004, Cisco Systems, Inc. All rights reserved.
Clock Synchronization and Transit Time Estimation
• The ICMP timestamp request message allows a host to ask for the current time according to the remote host.
• The remote host uses an ICMP timestamp reply message to respond to the request.
151515© 2004, Cisco Systems, Inc. All rights reserved.
Address Mask Requirements
• When a network administrator uses the process of subnetting to divide a major IP address into multiple subnets, a new subnet mask is created.
161616© 2004, Cisco Systems, Inc. All rights reserved.
Router Discovery Message
• When a host on the network boots, and the host has not been manually configured with a default gateway, it can learn of available routers through the process of router discovery.
171717© 2004, Cisco Systems, Inc. All rights reserved.
Router Solicitation Message
• A host generates an ICMP router solicitation message in response to a missing default gateway.
181818© 2004, Cisco Systems, Inc. All rights reserved.
Congestion and Flow Control Messages
ICMP source-quench messages are used to reduce the amount of data lost from congestion.
191919© 2004, Cisco Systems, Inc. All rights reserved.
Summary
20© 2004, Cisco Systems, Inc. All rights reserved.
Basic Router Troubleshooting
212121© 2004, Cisco Systems, Inc. All rights reserved.
Objectives
222222© 2004, Cisco Systems, Inc. All rights reserved.
The show ip route Command
The show ip route command displays the contents of the IP routing table.
232323© 2004, Cisco Systems, Inc. All rights reserved.
Static Routing
242424© 2004, Cisco Systems, Inc. All rights reserved.
Dynamic Routing
252525© 2004, Cisco Systems, Inc. All rights reserved.
Default Route
262626© 2004, Cisco Systems, Inc. All rights reserved.
Configuring a Default Route
272727© 2004, Cisco Systems, Inc. All rights reserved.
Determining Route Source and Destination
282828© 2004, Cisco Systems, Inc. All rights reserved.
Determining L2 and L3 Addresses
292929© 2004, Cisco Systems, Inc. All rights reserved.
Determining Administrative Distance
303030© 2004, Cisco Systems, Inc. All rights reserved.
Determining the Route Metric
Routing protocols use metrics to determine the best route to a destination.
313131© 2004, Cisco Systems, Inc. All rights reserved.
Determining the Route Next Hop
• Destination next hop associations determine the best path and which router to forward the packet to next. This router represents the next hop on the way to the final destination.
323232© 2004, 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
333333© 2004, 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.
343434© 2004, Cisco Systems, Inc. All rights reserved.
Introduction to Network Testing
353535© 2004, Cisco Systems, Inc. All rights reserved.
Structured Approach to Troubleshooting
363636© 2004, 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
373737© 2004, 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
383838© 2004, Cisco Systems, Inc. All rights reserved.
• Routing protocol not enabled • Wrong routing protocol
enabled • Incorrect IP addresses • Incorrect subnet masks
Typical Layer 3 Errors
393939© 2004, Cisco Systems, Inc. All rights reserved.
Layer 1 Problems in a Network
404040© 2004, Cisco Systems, Inc. All rights reserved.
Layer 3 Troubleshooting Using Ping
414141© 2004, Cisco Systems, Inc. All rights reserved.
Layer 7 Troubleshooting Using Telnet
424242© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Layer 1 Using show interfaces Command
The show interfaces serial command
434343© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Layer 2 Using show interfaces Command
444444© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Using show cdp neighbors Command
454545© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Using show cdp neighbors detail Command
464646© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Using traceroute Command
474747© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Routing Issues
The show ip route Command
484848© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Routing Issues
The show ip protocols Command
494949© 2004, Cisco Systems, Inc. All rights reserved.
Troubleshooting Using show controllers serial Command
The show controllers serial Command
505050© 2004, Cisco Systems, Inc. All rights reserved.
Introduction to debug
Debug syntax
515151© 2004, Cisco Systems, Inc. All rights reserved.
Summary
525252© 2004, Cisco Systems, Inc. All rights reserved.
Question/Answer