1. Troubleshooting an
Enterprise Network
Introducing Routing and Switching in the Enterprise –
Chapter 9
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2. Objectives
 Explain the importance of uptime and the types of
issues that cause failure.
 Isolate and correct switching problems.
 Isolate and correct routing issues.
 Isolate and correct WAN configurations.
 Isolate and correct ACL issues.
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3. Explain the Importance of Uptime and the Types
of Issues that Cause Failure
 Reduction in uptime can have a negative impact on a
business
 Downtime results in lost productivity and customer
frustration
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4. Explain the Importance of Uptime and the Types
of Issues that Cause Failure
Ways to ensure uptime:
 Proactive maintenance
 Network monitoring
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5. Explain the Importance of Uptime and the Types
of Issues that Cause Failure
 Failure domain: area of the network affected by the
failure or misconfiguration of a network device
 Troubleshoot resources with a larger failure domain
first, unless another problem is business critical
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6. Explain the Importance of Uptime and the Types
of Issues that Cause Failure
Troubleshooting approaches:
 Top-down
 Bottom-up
 Divide and conquer
 Trial and error
 Substitution
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7. Isolate and Correct Switching Problems
 The most common problems with switches occur at the
Physical Layer
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8. Isolate and Correct Switching Problems
 Suboptimal switching can be traced to STP problems
 Determine the VLAN assignment of non-functioning
ports
 Verify the trunking protocol if inter-VLAN routing is
required
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9. Isolate and Correct Switching Problems
 Verify VTP domain name
 Verify identical revision numbers on all devices
 Verify identical VTP versions on all devices
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10. Isolate and Correct Routing Issues
Determining and correcting RIP issues:
 Compatibility between RIPv1 and RIPv2
 Missing or incorrect network statements
 Errors in interface configurations
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11. Isolate and Correct Routing Issues
Determine and correct EIGRP issues:
 Mismatched AS numbers
 Errors in network statements
 Errors in interface addressing
 Auto-summarization used with discontiguous subnets
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12. Isolate and Correct Routing Issues
Determine and correct OSPF issues:
 Mismatched authentication or timer intervals
 Errors in wildcard masks or network statements
 Errors in interface configurations
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13. Isolate and Correct Routing Issues
 Determine and correct route redistribution
 Verify that users on internal routers can access external
networks
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14. Isolate and Correct WAN Configurations
Determine and correct WAN Physical Layer connection
issues:
 Clocking
 Cable types
 Loose or faulty
connectors
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15. Isolate and Correct WAN Configurations
Determine and correct WAN Data Link Layer and
Network Layer connection issues:
 Encapsulation mismatches
 Incompatible formats
 IP addressing errors
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16. Isolate and Correct WAN Configurations
 Determine and correct authentication issues
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17. Isolate and Correct ACL Issues
 Verify network connectivity before an ACL is applied
 Enable logging
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18. Isolate and Correct ACL Issues
Determine and correct ACL configuration and placement
issues:
 ACL statements should permit highest volume traffic
early in the ACL
 Implicit deny may have unintended consequences
 Improper ACL placement may waste bandwidth
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19. Summary
 Adherence to the three-layer hierarchical network
design model assists in troubleshooting
 When troubleshooting a network, determine the scope
of the problem and isolate the issue to a specific failure
domain.
 The most common problems with switches occur at the
Physical Layer.
 Use debug commands to isolate problems, not to
monitor normal network operation.
 When troubleshooting PPP, verify that the LCP has
been opened, authentication and NCP completed.
 ACLs can create complications in network operations.
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20. © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 20