CIT 384: Network Administration Slide #1

CIT 384: Network Administration

Troubleshooting Switches

CIT 384: Network Administration Slide #2

Topics

1. Troubleshooting

2. Physical Layer Troubleshooting

3. Cisco Discovery Protocol (CDP)

4. Ethernet Troubleshooting

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Troubleshooting

Novice1. Change something (don’t document.)

2. If it’s not fixed, go back to step 1.

Serial substitutionReplace each component in system with known good one

until system works.

Methodical1. Understand the problem.

2. Form hypotheses about possible causes.

3. Focus on most probable cause first.

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Cisco Troubleshooting Method

Define the Problem

Gather Facts

Create an Action Plan

Consider Possibilities

Observe Results

Implement the Action Plan

Document Results

Do ProblemSymptoms

Stop?

Finished

Start

Yes

No

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Define the Problem

Write description of problem and symptoms.– Writing forces you to clarify problem.– How does current situation differ from normal?

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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?

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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.

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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? ...

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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– Document results.– Verify that users see that problem is fixed.

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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.

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Troubleshooting Lower Layers• Electrical problems.• Cable problems.• Interface problems.• NIC configuration errors.• Switch config errors.

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Physical Layer Troubleshooting

1. 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.)

2. Use a cable tester.

3. Check for interface configuration errors.

4. Swap NIC for a known good NIC.

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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.

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Cable TestsContinuity

– 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.

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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.

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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.

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CDP Demo

• Local switch

• Lab switch

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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.)

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Interface Status CodesLine 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

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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

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Interface CountersFastEthernet0/1 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

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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.

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Interface Demo

• Local switch

• Lab switch

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References1. James Boney, Cisco IOS in a Nutshell, 2nd edition,

O’Reilly, 2005. 2. Cisco, Cisco Connection Documentation,

http://www.cisco.com/univercd/home/home.htm3. Cisco, Internetwork Troubleshooting Handbook,

http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/index.htm, 2006.

4. Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.

5. Priscilla Oppenheimer and Joseph Bardwell, Troubleshooting Campus Networks, Addison-Wesley, 2002.

6. W. Richard Stevens, TCP/IP Illustrated, Addison-Wesley, 1994.