1. CIT 384: Network Administration
Troubleshooting Switches
CIT 384: Network Administration

2. CIT 384: Network Administration
Topics
Troubleshooting
Physical Layer Troubleshooting
Cisco Discovery Protocol (CDP)
Ethernet Troubleshooting
CIT 384: Network Administration

3. CIT 384: Network Administration
Troubleshooting
Novice
Change something (don’t document.)
If it’s not fixed, go back to step 1.
Serial substitution
Replace each component in system with known good one until system works.
Methodical
Understand the problem.
Form hypotheses about possible causes.
Focus on most probable cause first.
CIT 384: Network Administration

4. Cisco Troubleshooting Method
Start
Define the Problem
Gather Facts
Finished
Consider Possibilities
Document Results
Create an Action Plan
Yes
Implement the Action Plan
Do Problem
Symptoms
Stop?
Observe Results
CIT 384: Network Administration No

5. CIT 384: Network Administration
Define the Problem
Write description of problem and symptoms.
Writing forces you to clarify problem.
How does current situation differ from normal?
CIT 384: Network Administration

6. CIT 384: Network Administration
Gather Facts
Facts include
Data from users + admins.
Data from protocol analyzer, IOS diagnostics.
Answer these questions
How often does problem occur?
When did problem first occur?
What changes were made right before problem started happening?
Is the problem reproducible?
CIT 384: Network Administration

7. Consider Possibilities
Isolate the problem based on facts
Which devices are having problems?
At which network layer is the problem?
Which protocols are showing problems?
Determine possibilities
Have you seen this problem before?
Have you seen a similar problem before?
Use your TCP/IP knowledge and facts to determine what might fail.
CIT 384: Network Administration

8. CIT 384: Network Administration
Create an Action Plan
Develop plan to test likely causes.
Change only one variable at a time.
Otherwise you don’t know what fixed it.
Divide and conquer
Partition problem domain into discrete areas that are physically or logically isolated.
Testing outward
Does local NIC work?
Can you communicate with PC on same subnet?
Can you communicate with router?
Can you communicate with next hop? ...
CIT 384: Network Administration

9. Implement and Observe Results
Follow action plan steps
Document which step you’re trying.
Document results.
Test all fixes you make.
Be sure there are no side-effects.
Observing results
Verify that users see that problem is fixed.
CIT 384: Network Administration

10. CIT 384: Network Administration
Document Results
Record which plan worked and why.
Ensures that you can fix the problem again.
If your fix causes new problems later, you know what you did and how to undo it.
CIT 384: Network Administration

11. Troubleshooting Lower Layers
Electrical problems.
Cable problems.
Interface problems.
NIC configuration errors.
Switch config errors.
Application
Presentation
Session
Transport
Network
Data Link
Physical
CIT 384: Network Administration

12. Physical Layer Troubleshooting
Check the link lights
NICs have transmit, receive, collision LEDs.
Switches/routers have many more LEDs.
Lights blink time is much longer than actual event (at 10Mbps, 1 byte transferred per us.)
Use a cable tester.
Check for interface configuration errors.
Swap NIC for a known good NIC.
CIT 384: Network Administration

13. CIT 384: Network Administration
Cable Testers
Wide variety of testers exist.
Specialized for different media types (Ethernet, fiber, etc.)
More capabilities mean higher prices, starting around $100 to many $1000s.
Fluke NetTool Series II image from
CIT 384: Network Administration

14. CIT 384: Network Administration
Cable Tests
Continuity
Tries to pass a current down the cable. If the current doesn’t flow, cable is bad (short, etc.)
Attenuation
How much signal is lost over cable length. High values indicate wrong cable type, bad connector, excessive length.
Length
By timing return of signal (signal on UTP at 0.59c), it determines the length of the cable.
Wire map
Checks if pins on each end are correctly paired.
Near End Cross-Talk (NEXT)
Measure how much signal on one wire interferes with other wires. High values can indicate improper termination or wrong cable type.
CIT 384: Network Administration

15. Cisco Discovery Protocol (CDP)
Discovers info about neighboring devices
Device identifier (hostname)
Address list
Local interface
Port identifier
Capabilities list
Platform (hardware + software versions)
Routers and switches advertise info by multicasting CDP messages.
CIT 384: Network Administration

16. CIT 384: Network Administration
CDP Commands
Command
Description
show cdp neighbors
List one summary line of info about each neighbor.
show cdp neighbors detail
List one set of info about each neighbor.
show cdp entry name
List same info as above command but only for the named neighbor.
CIT 384: Network Administration

17. CIT 384: Network Administration
CDP Demo
Local switch
Lab switch
CIT 384: Network Administration

18. CIT 384: Network Administration
Interface Status
show interfaces description
Lists line and protocol status (up/down)
Switch will only forward frames in up/up state.
show interfaces status
One-line summary of each interface’s status.
Status (connected or notconnect)
Duplex(auto, a-full, a-half, full, half)
Speed(10, 100, 1000, a-)
Type(10/100BaseTX, etc.)
CIT 384: Network Administration

19. Interface Status Codes
Line Status
Protocol Status
Interface Status
Typical Cause
Administratively down
Down
disabled
Interface configured with shutdown command.
notconnect
No cable; bad or wrong cable; other end is down. Up
Not expected.
down
(err-disabled)
err-disabled
Port security has disabled interface.
connect
Interface working
CIT 384: Network Administration

20. CIT 384: Network Administration
Interface Status
show interfaces name
Hardware (MAC address)
Speed and duplex settings
Flow control
ARP
Statistics
Input rate: bits/sec, packets/sec
Output rate: bits/sec, packets/sec
Total packets, bytes, broadcasts, collisions
Various error types
CIT 384: Network Administration

21. CIT 384: Network Administration
Interface Counters
FastEthernet0/1 is up, line protocol is up (connected)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
packets input, bytes, 0 no buffer
Received broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, multicast, 0 pause input
0 input packets with dribble condition detected
packets output, bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
CIT 384: Network Administration

22. Interface Counter Errors
Problem
Counter Values
Common Causes
Excessive noise
Many input errors,
few collisions.
Wrong cable category; damaged cables; EMI.
Collisions
More than 0.1% of frames are collisions.
Duplex mismatch (seen on half-duplex side; jabber (NIC ignores Ethernet rules); DoS
Late collisions
Increasing late collisions
Collision domain or cable too long; duplex mismatch.
CIT 384: Network Administration

23. CIT 384: Network Administration
Interface Demo
Local switch
Lab switch
CIT 384: Network Administration

24. CIT 384: Network Administration
References
James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.
Cisco, Cisco Connection Documentation,
Cisco, Internetwork Troubleshooting Handbook,
Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.
Priscilla Oppenheimer and Joseph Bardwell, Troubleshooting Campus Networks, Addison-Wesley, 2002.
W. Richard Stevens, TCP/IP Illustrated, Addison-Wesley, 1994.
CIT 384: Network Administration