Lesson 4 - Troubleshooting Ethernet Networks

Question 1

What is the first step in the CompTIA® Network+® troubleshooting methodology?

Answer 1

1) Identify the problem:

• Gather information.
• Duplicate the problem, if possible.
• Question users.
• Identify symptoms.
• Determine if anything has changed.
• Approach multiple problems individually.

Question 2

What is the second step in the CompTIA® Network+® troubleshooting methodology?

Answer 2

2) Establish a theory of probable cause:

Question the obvious.
Consider multiple approaches.
Top-to-bottom/bottom-to-top OSI model.
Divide and conquer.

Question 3

What is the third step in the CompTIA® Network+® troubleshooting methodology?

Answer 3

3) Test the theory to determine cause:

Once theory is confirmed, determine next steps to resolve problem.
If theory is not confirmed, reestablish new theory or escalate.

Question 4

What is the forth step in the CompTIA® Network+® troubleshooting methodology?

Answer 4

4) Establish a plan of action to resolve the problem and identify potential effects.

Question 5

What is the fifth step in the CompTIA® Network+® troubleshooting methodology?

Answer 5

5) Implement the solution or escalate as necessary.

Question 6

What is the sixth step in the CompTIA® Network+® troubleshooting methodology?

Answer 6

6) Verify full system functionality, and if applicable, implement preventive measures.

Question 7

What is the seventh step in the CompTIA® Network+® troubleshooting methodology?

Answer 7

7) Document findings, actions, and outcomes.

Question 8

Explain a loopback adapter

Answer 8

A network loopback adapter (or loopback plug) is a specially wired RJ-45 plug with a 6” stub of cable. The wiring pinout is pin 1 (Tx) to pin 3 (Rx) and pin 2 (Tx) to pin 6 (Rx). This means that the packet sent by the NIC is received by itself. This is used to test for bad ports and network cards.

Question 9

what does solid green indicate on a switch LED status indicator?

Answer 9

Solid green-The link is connected but there is no traffic.

Question 10

what does flickering green indicate on a switch LED status indicator?

Answer 10

Flickering green-The link is operating normally (with traffic). The blink rate indicates the link speed.

Question 11

what does no light indicate on a switch LED status indicator?

Answer 11

No light- The link is not working or the port is shut down.

Question 12

what does blinking amber indicate on a switch LED status indicator?

Answer 12

Blinking amber- A fault has been detected (duplex mismatch, excessive collisions, or redundancy check errors, for instance).

Question 13

what does solid amber indicate on a switch LED status indicator?

Answer 13

Solid amber- The port is blocked by the spanning tree algorithm, which works to prevent loops within a switched network.

Question 14

what do you do next if there are no obvious hardware failures?

Answer 14

you should verify the settings on the switch port and NIC. Most adapters and switches successfully autonegotiate port settings. If this process fails, the adapter and port can end up with mismatched speed or duplex settings. In most cases, this will be because either the adapter or the switch port has been manually configured. If a host is set to a fixed configuration and the switch is set to autonegotiate, the switch will default to 10 Mbps/half-duplex because the host will not negotiate with it! So, if the host is manually configured to 100 Mbps/full duplex, the link will fail. Setting both to autonegotiate will generally solve the problem. A speed mismatch will cause the link to fail, while a duplex mismatch will slow the link down (it will cause high packet loss and late collisions).

Question 15

What is TDR?

Answer 15

A cable tester might incorporate the function of a time domain reflectometer (TDR). A TDR is used to measure the length of a cable run and can locate open and short circuits, kinks/sharp bends, and other imperfections in cables that could affect performance. A TDR transmits a short signal pulse of known amplitude and duration down a cable and measures the corresponding amplitude and time delay associated with resultant signal reflections. A TDR analyzes these reflections and can display any problems found and their location. The TDR measures the amount of time taken for the signal to bounce back and can therefore calculate the distance to the cable fault to within a meter, which makes isolating the problem simpler.

Question 16

Wire map testers can identify what problems?

Answer 16

• Continuity (open)-A conductor does not form a circuit because of cable damage or because the connector is not properly wired.
  Short-Two conductors are joined at some point, usually because the insulating wire is damaged, or a connector is poorly wired.
• Incorrect pin-out/incorrect termination/mismatched standards-The conductors are incorrectly wired into the terminals at one or both ends of the cable. The following transpositions are common:
• Reversed pair-The conductors in a pair have been wired to different terminals (for example, from pin 3 to pin 6 and pin 6 to pin 3 rather than pin 3 to pin 3 and pin 6 to pin 6).
• Crossed pair (TX/RX reverse)-The conductors from one pair have been connected to pins belonging to a different pair (for example, from pins 3 and 6 to pins 1 and 2). This may be done deliberately to create a crossover cable, but such a cable would not be used to link a host to a switch.
• Split pair. This is where both ends of a single wire in one pair are wired to terminals belonging to a different pair. This type of fault can only be detected by a cable tester that measures crosstalk.

Question 17

What happens if a cable link is too long?

Answer 17

If a cable link is too long, decibel (dB) loss (or insertion loss) may mean that the link experiences problems with high error rates and retransmissions (frame or packet loss) resulting in reduced speeds and possibly loss of connectivity.

Question 18

What are the types of crosstalk that can be measured?

Answer 18

• Near End (NEXT)-This measures crosstalk on the receive pairs at the transmitter end and is usually caused by excessive untwisting of pairs or faulty bonding of shielded elements.
• Attenuation to Crosstalk Ratio, Near End (ACRN)-This is the difference between insertion loss and NEXT. ACR is equivalent to a signal-to-noise ratio (SNR). A high value means that the signal is stronger than any noise present; a result closer to 0 means the link is likely to be subject to high error rates.
• Attenuation-to-Crosstalk Ratio, Far End (ACRF)-Far End Crosstalk (FEXT) is measured on the receive pairs at the recipient end. The difference between insertion loss and FEXT gives ACRF, which measures cable performance regardless of the actual link length.
• Power sum - Gigabit and 10 GbE Ethernet use all four pairs. Power sum crosstalk calculations (PSNEXT, PSACRN, and PSACRF) confirm that a cable is suitable for this type of application. They are measured by energizing three of the four pairs in turn.
• Complete loss of connectivity indicates a break in the cable (or a completely faulty installation), while intermittent loss of connectivity is more likely to be caused by attenuation, crosstalk, or noise.

Question 19

What are the two main formats for patch cords?

Answer 19

Straight through and crossover.

Question 20

Describe a straight through cable

Answer 20

Straight through-the cable is terminated with either T568A at both ends or T568B at both ends. This type of cable is used for an uplink (MDI port to MDIX port).

Question 21

Describe a crossover cable

Answer 21

Crossover-the cable would have from one side to another: pin 1 to pin 3, pin 2 to pin 6, pin 3 to pin 1, pin 4 to pin 7, pin 5 to pin 8, pin 6 to pin 2, pin 7 to pin 4, and pin 8 to pin 5. This type of cable is used to connect an end system (host) to another host or a hub to a hub.

Question 22

What is a rollover cable?

Answer 22

A rollover/console cable is used to connect a PC or laptop to the command line terminal of a switch or router. The console port connection on the appliance is a standard RJ-45 jack (but wired in a different way to Ethernet). A legacy console cable has a serial DB-9 connector for the PC end. As almost no computers come with DB-9 ports anymore, modern cables use a USB connector and port. Console cable is traditionally colored pale blue.

Question 23

what kind of ethernet cable is required to support PoE?

Answer 23

Cat 3 or better

Question 24

what kind of ethernet cable is required to support PoE+?

Answer 24

Cat 5e or better

Question 25

How can the location of a break be found in a fiber line?

Answer 25

optical time domain reflectometer (OTDR). This sends light pulses down the cable and times how long it takes for any reflections to bounce back from the break. A broken cable will need to be repaired (spliced) or replaced. An OTDR can also be used to verify that new splices are sound.

Question 26

How do you ensure that each fiber channel has sufficient power?

Answer 26

An optical spectrum analyzer (OSA) is typically used with wavelength division multiplexing (WDM) to ensure that each channel has sufficient power.