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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 3

4.
Explain the Importance of Uptime and the Types
of Issues that Cause Failure
Ways to ensure uptime:
 Proactive maintenance
 Network monitoring
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 4

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 5

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 6

7.
Isolate and Correct Switching Problems
 The most common problems with switches occur at the
Physical Layer
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 7

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 8

9.
Isolate and Correct Switching Problems
 Verify VTP domain name
 Verify identical revision numbers on all devices
 Verify identical VTP versions on all devices
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 9

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 10

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 11

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 12

13.
Isolate and Correct Routing Issues
 Determine and correct route redistribution
 Verify that users on internal routers can access external
networks
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 13

14.
Isolate and Correct WAN Configurations
Determine and correct WAN Physical Layer connection
issues:
 Clocking
 Cable types
 Loose or faulty
connectors
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 14

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 15

16.
Isolate and Correct WAN Configurations
 Determine and correct authentication issues
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 16

17.
Isolate and Correct ACL Issues
 Verify network connectivity before an ACL is applied
 Enable logging
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 17

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
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 18

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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 19

20.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 20