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Comptia network + > 5.3 Network Troubleshooting and ToolsGiven a scenario, troubleshoot common wired connectivity and performance issues. > Flashcards

5.3 Network Troubleshooting and ToolsGiven a scenario, troubleshoot common wired connectivity and performance issues. Flashcards

1
Q

Signal loss

A

• Usually gradual• Signal strength diminishes over distance• Attenuation• Loss of intensity as signal moves through a medium• Electrical signals through copper, light through fiber• Radio waves through the air

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2
Q

Decibels (dB)

A 
Signal strength ratio measurements
• One-tenth of a bel
• Capital B for Alexander Graham Bell
• Logarithmic scale
• Add and subtract losses and gains
• 3 dB = 2x the signal
• 10 dB = 10x the signal
• 20 dB = 100x the signal
• 30 db = 1000x the signal
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3
Q

dB loss symptoms

A

No connectivity• No signal!• Intermittent connectivity• Just enough signal to sync the link• Poor performance• Signal too weak• CRC errors, data corruption• Test each connection• Test distance and signal loss

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4
Q

Latency

A

• A delay between the request and the response• Waiting time• Some latency is expected and normal• Laws of physics apply• Examine the response times at every step along the way• This may require multiple measurement tools• Packet captures can provide detailed analysis• Microsecond granularity• Get captures from both sides

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5
Q

Jitter

A

Jitter is the time between frames

Excessive jitter can cause you to miss information, “choppy” voice calls

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6
Q

Troubleshooting excessive jitter

A 
Confirm available bandwidth
Make sure the infrastructure is working as expected
Apply QoS (Quality of Service)
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7
Q

Crosstalk (XT)

A 
Signal on one circuit affects another circuit
Leaking of signal
Measure XT with cable testers
Near End Crosstalk (NEXT)
Far End Crosstalk (FEXT)
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8
Q

Troubleshooting crosstalk

A 
Almost always a wiring issue
Check your crimp
• Maintain your twists
• The twist helps to avoid crosstalk
• Category 6A increases cable diameter
• Increased distance between pairs
• Test and certify your installation
• Solve problems before they are problems
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9
Q

Avoiding EMI and interference

A

Electromagnetic interference
Cable handling
EMI and interference with copper cables
Test after installation

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10
Q

Opens and shorts

A 
A short circuit
• Two connections are touching
• Wires inside of a cable or connection
• An open circuit
• A break in the connection
• Complete interruption
• Can be intermittent
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11
Q

Troubleshooting opens and shorts

A

May be difficult to find
• The wire has to be moved just the right way
• Wiggle it here and there
• Replace the cable with the short or open
• Difficult or impossible to repair
• Advanced troubleshooting with a TDR
• Time Domain Reflectometer

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12
Q

Troubleshooting pin-outs

A
  • Cables can foul up a perfectly good plan
  • Test your cables prior to implementation
  • Many connectors look alike
  • Do you have a good cable mapping device?
  • Get a good cable person
  • It’s an art
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13
Q

T568A and T568B termination

A

Pin assignments from EIA/TIA-568-B standard
• Eight conductor 100-ohm balanced twisted-pair cabling
• T568A and T568B are different pin assignments for 8P8C connectors
• Assigns the T568A pin-out to horizontal cabling
• Many organizations traditionally use 568B
• Difficult to change in mid-stream
• You can’t terminate one side of the cable with568A and the other with 568B
• It won’t be a straight-through cable

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14
Q

Incorrect cable type

A

Excessive physical errors, CRC errors
• Check your layer 1 first
• Check the outside of the cable
• Usually printed on the outside
• May also have length marks printed
• Confirm the cable specifications with a TDR
• Advanced cable tester can identify damaged cables

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15
Q

Troubleshooting interfaces

A 
Interface errors
• May indicate bad cable or hardware problem
• Verify configurations
• Speed, duplex, VLAN, etc
• Verify two-way traffic
• End-to-end connectivity
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16
Q

Transceiver mismatch

A
  • Transceivers have to match the fiber
  • Single mode transceiver connects to single mode fiber
  • Transceiver needs to match the wavelength
  • 850nm, 1310nm, etc.
  • Use the correct transceivers and optical fiber
  • Check the entire link
  • Signal loss
  • Dropped frames, missing frames
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17
Q

Reversing transmit and receive

A 
Wiring mistake• Cable ends• Punchdowns
• Easy to find with a wire map
• 1-3, 2-6, 3-1, 6-2
• Simple to identify
• Some network interfaces will automatically correct (Auto-MDIX
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18
Q

TX/RX reversal troubleshooting

A 
No connectivity
• Auto-MDIX might connect
• Try turning it on
• Locate reversal location
• Often at a punchdown
• Check your patch panel
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19
Q

Damaged cables

A

Copper cables are pretty rugged
• But they aren’t indestructible
• Cables can be out in the open
• Stepped on, folded between a table and wall
• Check your physical layer
• Cables should not be bent or folded
• Check for any bent pins on the device
• It’s difficult to see inside of the cable
• Check your TDR, replace the cable (if possible)

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20
Q

Bottlenecks

A
  • There’s never just one performance metric
  • A series of technologies working together
  • I/O bus, CPU speed, storage access speed, network throughput, etc.
  • One of these can slow all of the others down
  • You must monitor all of them to find the slowest one
  • This may be more difficult than you might expect
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21
Q

Interface configuration problems

A 
Poor throughput
• Very consistent, easily reproducible
• No connectivity
• No link light
• No connectivity
• Link light and activity light
22
Q

Interface configuration

A
  • Auto vs. Manual configuration
  • Personal preference
  • Light status
  • No light, no connection
  • Speed
  • Must be identical on both sides
  • Duplex
  • If mismatched, speed will suffer
23
Q

VLAN mismatch

A
  • Switch is configured with the incorrect VLAN
  • Configured per switch interface
  • Link light, but no surfing
  • A DHCP IP address may not be on the correct subnet
  • Manually IP addressing won’t work at all
  • Check the switch configuration for VLAN configuration
  • Each port should have a VLAN setting
  • VLAN 1 is usually the defaul
24
Q

Duplex/speed match

A

Speed and duplex
• Speed: 10 / 100 /1,000 / Auto
• Duplex: Half / Full / Auto
• Incorrect speed
• Many switch configurations will auto-negotiate speed
• Less than expected throughput
• Incorrect duplex
• Again, the switch may auto-negotiate
• Needs to match on both sides
• A mismatch will cause significant slowdowns
• Increase in Late Collisions may indicate a duplex mismatch

25
Q

Reflection

A
  • Wireless signals can bounce off some surfaces
  • Depends on the frequencies and the surfaces
  • Too much reflection can weaken the signal
  • A little multipath interference actually helps with MIMO
  • Position antennas to avoid excessive reflection
  • May not be a problem for MIMO in 802.11n and 802.11ac
26
Q

Refraction

A
  • Signal passes through an object and exits at a different angle
  • Similar to light through water
  • Data rates are affected - Signal is less directional
  • Outdoor long-distance wireless links
  • Changes in air temperature and water vapor
27
Q

Absorption

A
  • Signal passes through an object and loses signal strength
  • Especially through walls and windows
  • Different objects absorb differently as frequencies change
  • 2.4 GHz may have less absorption than 5 GHz
  • Put the antennas on the ceiling
  • And avoid going through walls
28
Q

Latency and jitter

A

• Latency - Delays between transmission and reception
• Jitter - Deviation from a predictable data stream
• Wireless interference and signal issues
• Slower data rates
• Increase in retransmission’s
Capacity issues

29
Q

Attenuation

A
  • Wireless signals get weaker as you move farther from the antenna
  • The attenuation can be measured with a Wi-Fi analyzer
  • Control the power output on the access point
  • Not always an option
  • Use a receive antenna with a higher gain
  • Capture more of the signal
  • Move closer to the antenna - May not be possible
30
Q

Interference

A 
Interference
• Something else is using our frequency
• Predictable
• Florescent lights, microwave ovens, cordless telephones, high-power sources
• Unpredictable - Multi-tenant building
• Measurements
• netstat –e
• Performance Monitor
31
Q

Incorrect antenna type

A 
The antenna must fit the room
• Or the distance between sender and receiver
• Omnidirectional
• Useful on the ceiling
• Not very useful between buildings
• Directional
• Used often between two points
• Or on a wall-mounted access point
• The access point may provide options
• Connect different antennas
32
Q

Incorrect antenna placement

A
  • Interference• Overlapping channels
  • Slow throughput
  • Data fighting to be heard through the interference
  • Check access point locations and channel settings
  • A challenge for 2.4 GHz
  • Much easier for 5 GHz
33
Q

Overcapacity

A
  • Device saturation
  • Too many devices on one wireless network
  • There are only so many frequencies
  • The 5 GHz can really help with this
  • Bandwidth saturation
  • Large data transfers
  • Common in large meeting places
  • Conferences• Airports• Hotels
34
Q

Frequency mismatch

A

Devices have to match the access point• 2.4 GHz, 5 GHz
• Verify the client is communicating over the correct channel
• This is normally done automatically• May not operate correctly if manually configured
• Older standards may slow down the newer network
• 802.11b compatibility mode on 802.11n networks
• Every access point has an SSID
• But did you connect to the right one?• This can be more confusing than you might think• Public Wi-Fi Internet• Guest Internet• Internet
• Confirm the correct SSID settings• Should be listed in the current connection status

35
Q

Wrong passphrase

A

Wireless authentication
• Many different methods
• Required to connect to the wireless network
• If not connected, check the authentication
• Shared passphrase
• Common in a SOHO, not in the enterprise
• 802.1X
• Used for the enterprise
• Make sure the client is configured to use 802.1X

36
Q

Security type mismatch

A
  • Encryption on wireless is important• Make sure the client matches the access point
  • This is much easier these days• Almost everything is at the level of WPA2
  • Some legacy equipment may not be able to keep up• If you change the access point, you may not be able to support it
  • Migrate all of your WEP to WPA2• And any WPA
37
Q

Signal to noise ratio

A
  • Signal• What you want
  • Noise• What you don’t want• Interference from other networks and devices
  • You want a very large ratio• The same amount of signal to noise (1:1) would be bad
38
Q

Duplicate IP addresses

A
  • Static address assignments - Must be very organized
  • DHCP isn’t a panacea• Static IP addressing• Multiple DHCP servers overlap• Rogue DHCP servers
  • Intermittent connectivity• Two addresses “fight” with each other
  • Blocked by the OS - Checks when it starts
39
Q

Troubleshooting duplicate IP addresses

A

Check your IP addressing - Did you misconfigure?• Ping an IP address before static addressing• Does it respond?
• Determine the IP addresses
• Ping the IP address, check your ARP table• Find the MAC address in your switch MAC table
• Capture the DHCP process• What DHCP servers are responding?

40
Q

Duplicate MAC addresses

A
  • Not a common occurrence• MAC addresses are designed to be unique• May be a man-in-the-middle attempt
  • Mistakes can happen• Locally administered MAC addresses• Manufacturing error
  • Intermittent connectivity• Confirm with a packet capture, should see ARP contention
  • Use the ARP command from another computer• Confirm the MAC matches the IP
41
Q

Expired IP addresses

A

A DHCP address should renew well before the lease expires
Client gives up the IP address at the end of the lease APIPA address is assigned• Checks in occasionally for a DHCP server• Look for an APIPA assigned address• 169.254..• Check the status of your DHCP server

42
Q

Rogue DHCP server

A

IP addresses assigned by a non-authorized server• There’s no inherent security in DHCP• Client is assigned an invalid or duplicate address• Intermittent connectivity, no connectivity• Disable rogue DHCP communication• Enable DHCP snooping on your switch• Authorized DHCP servers in Active Directory• Disable the rogue• Renew the IP leases

43
Q

Untrusted SSL certificate

A

Browsers trust signatures from certain CAs• A certificate was signed by a CA that’s not in our list• Error message on the browser• Certificate Authority Invalid• Check the certificate details• Look for the issuing CA• Compare to the CA list on your computer• If it’s an internal server, it may be internally signed• Add your internal CA certificate to the list

44
Q

Incorrect time

A

• Some cryptography is very time sensitive• Active Directory requires clocks set within five minutes of each other• Kerberos communication uses a time stamp• If the ticket shown during authentication is too old, it’s invalid• Client can’t login• Check the timestamp of the client and the server• Configure NTP on all devices• Automate the clock setting

45
Q

Exhausted DHCP scope

A

Client received an APIPA address• Local subnet communication only• Check the DHCP server• Add more IP addresses if possible• IP address management (IPAM) may help• Monitor and report on IP address shortages• Lower the lease time• Especially if there are a lot of transient users

46
Q

Blocked TCP/UDP ports

A

Applications not working• Slowdowns with other applications• Firewall or ACL configuration• Security choke points• Confirm with a packet capture• No response to requests• Run a TCP- or UDP-based traceroute tool• See how far your packet can go

47
Q

Incorrect host-based firewall setting

A

• Applications not working• Based on the application in use and not necessarily the protocol and port• Check the host-based firewall settings• Accessibility may be limited to an administrator• Managed from a central console• Take a packet capture• The traffic may never make it to the network• Dropped by the operating system

48
Q

Incorrect ACL setting

A

Only certain IP addresses accessible• Or none• Access Control Lists• IP address, port numbers, and other parameters• Can allow or deny traffic by filtering packets• Confirm with packet captures and TCP/UDP traceroutes• Identify the point of no return

49
Q

Unresponsive service

A

No response to an application request• No answer• Do you have the right port number?• And protocol (TCP/UDP)?• Confirm connectivity• Ping, traceroute• Is the application still working?• Telnet to the port number and see if it responds

50
Q

Hardware failure

A

• No response• Application doesn’t respond• Confirm connectivity• Without a ping, you’re not going to connect• Run a traceroute• See if you’re being filtered• Should make it to the other side• Check the server• Lights? Fire?

Comptia network + (26 decks)

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