CIT 384: Network Administration
Troubleshooting Switches
CIT 384: Network Administration
Slide #1
Topics
1.
2.
3.
4.
Troubleshooting
Physical Layer Troubleshooting
Cisco Discovery Protocol (CDP)
Ethernet Troubleshooting
CIT 384: Network Administration
Slide #2
Troubleshooting
Novice
1. Change something (don’t document.)
2. 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
1. Understand the problem.
2. Form hypotheses about possible causes.
3. Focus on most probable cause first.
CIT 384: Network Administration
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Cisco Troubleshooting Method
Start
Define the Problem
Finished
Gather Facts
Consider Possibilities
Document Results
Create an Action Plan
Yes
Implement the Action Plan
Do Problem
Symptoms
Stop?
Observe Results
CIT 384: Network Administration
No
Slide #4
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
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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
Slide #6
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
Slide #7
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
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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
Slide #8
Implement and Observe Results
Follow action plan steps
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–
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Document which step you’re trying.
Document results.
Test all fixes you make.
Be sure there are no side-effects.
Observing results
– Document results.
– Verify that users see that problem is fixed.
CIT 384: Network Administration
Slide #9
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
Slide #10
Troubleshooting Lower Layers
Application
Presentation
Session
Transport
•
•
•
•
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Electrical problems.
Cable problems.
Interface problems.
NIC configuration errors.
Switch config errors.
Network
Data Link
Physical
CIT 384: Network Administration
Slide #11
Physical Layer Troubleshooting
1. Check the link lights
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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.)
2. Use a cable tester.
3. Check for interface configuration errors.
4. Swap NIC for a known good NIC.
CIT 384: Network Administration
Slide #12
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.
CIT 384: Network Administration
Slide #13
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
Slide #14
Cisco Discovery Protocol (CDP)
Discovers info about neighboring devices
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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
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CDP Commands
Command
show cdp neighbors
show cdp neighbors
detail
show cdp entry name
Description
List one summary line of info
about each neighbor.
List one set of info about each
neighbor.
List same info as above
command but only for the
named neighbor.
CIT 384: Network Administration
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CDP Demo
• Local switch
• Lab switch
CIT 384: Network Administration
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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
Slide #18
Interface Status Codes
Line Status
Protocol Status
Interface Status Typical Cause
Administratively
down
Down
disabled
Interface
configured with
shutdown
command.
Down
Down
notconnect
No cable; bad or
wrong cable;
other end is
down.
Up
Down
notconnect
Not expected.
Down
down
(err-disabled)
err-disabled
Port security has
disabled
interface.
Up
Up
connect
Interface
working
CIT 384: Network Administration
Slide #19
Interface Status
show interfaces name
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Hardware (MAC address)
Speed and duplex settings
Flow control
ARP
Statistics
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•
Input rate: bits/sec, packets/sec
Output rate: bits/sec, packets/sec
Total packets, bytes, broadcasts, collisions
Various error types
CIT 384: Network Administration
Slide #20
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
109212347 packets input, 70838129251 bytes, 0 no buffer
Received 308656 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 305530 multicast, 0 pause input
0 input packets with dribble condition detected
104860540 packets output, 64589349605 bytes, 0
underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
CIT 384: Network Administration
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Interface Counter Errors
Problem
Excessive noise
Counter Values
Many input errors,
few collisions.
Collisions
More than 0.1% of
frames are
collisions.
Late collisions
Increasing late
collisions
CIT 384: Network Administration
Common Causes
Wrong cable
category; damaged
cables; EMI.
Duplex mismatch
(seen on half-duplex
side; jabber (NIC
ignores Ethernet
rules); DoS
Collision domain or
cable too long;
duplex mismatch.
Slide #22
Interface Demo
• Local switch
• Lab switch
CIT 384: Network Administration
Slide #23
References
1.
2.
3.
4.
5.
6.
James Boney, Cisco IOS in a Nutshell, 2nd edition,
O’Reilly, 2005.
Cisco, Cisco Connection Documentation,
http://www.cisco.com/univercd/home/home.htm
Cisco, Internetwork Troubleshooting Handbook,
http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/
index.htm, 2006.
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
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