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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Network Troubleshooting
Chapter 21
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Objectives
• Describe appropriate troubleshooting tools
and their functions
• Analyze and discuss the troubleshooting
process
• Resolve common network issues
• Describe the Internet of Things
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Test Specific
Troubleshooting Tools
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Troubleshooting Tools
• Required tools vary depending on the job type
• Always consider the safety of your data first
– Before beginning troubleshooting
• Many software diagnostic tools are available
– You may not need any purchased software tools
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Hardware Tools
•
•
•
•
•
•
•
Cable testers, TDRs, and OTDRs
Certifiers
Voltage event recorders
Protocol analyzers
Cable strippers
Multimeters
Tone probes/generators
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Hardware Tools (cont’d.)
• Line testers
• Butt sets
• Punchdown tools
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Potential Circuit Problems
• Open circuit
– Cable wires do not connect from one end of the
cable to the other
• Short circuit
– Connection between wires in a cable
• Wire map problem
– Wires do not connect to the proper location on
the jack or plug
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Potential Circuit Problems (cont’d.)
• Crosstalk
– The electrical signal from one wire pair causes
interference in a nearby wire pair
• Noise
– Spurious signals typically due to faulty hardware
or poorly crimped jacks
• Impedance mismatch
– Can occur where cables of different types connect
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Cable Testers, TDRs, and ODTRs
• Cable testers
– Help identify continuity or wire map problems
• TDRs (time domain reflectometers)
– Locate copper cable breaks
• OTDRs (optical time domain reflectometers)
– Locate fiber-optic cable breaks
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.1 Typical cable tester
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.2 An EXFO AXS-100 OTDR (photo courtesy of EXFO)
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Certifiers
• Ensure a cable can handle its rated capacity
– Use when a cable is not moving data as it should
• Require a loopback on the far end
• Problems that reduce cable capacity
– Crosstalk
– Attenuation
– Interference
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Light Meter
• Measures light loss in a fiber optic cable
– Uses a high-powered light source and detector
• Also called optical power meter
• Inherent impurities in the glass can reduce
light transmission
– Other causes: dust, poor connections, and light
leakage
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.3 Fiberlink® 6650 Optical Power Meter
(photo courtesy of Communications Specialties, Inc.)
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Voltage Event Recorder/
Temperature Monitor
•
•
•
•
Detect power and heat problems
Symptom: intermittent problems
Possible cause: heat problems in server rooms
A voltage event recorder can help identify
problems with electricity
• A temperature monitor collects temperature
information over time
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Protocol Analyzers
• Monitor protocols running at different layers
– Application, Session, Transport, Network, and
Data Link
• May be hardware or software tools
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Protocol Analyzers (cont’d.)
• Problem symptoms
– A session fails to start
– A DNS server fails to respond
– Confusing information appears on the network
– You suspect a rogue DHCP server exists
– Excess or unexpected traffic slows the network
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Cable Strippers/Snips
• Needed for making UTP cables
– Also need crimpers
• Cable stripper and crimper can be combined
in a single tool
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.4 A cable stripping and crimping tool
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Multimeters
• Test AC and DC voltage, resistance, and
continuity
• Often used for continuity testing when a cable
tester is not available
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Tone Probes and Tone Generators
• Work together to help locate a particular cable
• A tone generator puts a signal (tone) on a wire
• A tone probe on opposite end detects the
signal
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Butt Sets
• A telephone person’s best friend
• Tap into a 66- or 110-block to test a line
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Line Testers
• Simple devices for checking telephone wiring
integrity
• Possible outcomes when checking a twisted
pair line
– Good
– Dead
– Reverse wired
– An AC voltage on the line
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Punchdown Tools
• Put UTP wires into 66- and 110-blocks
• Repunch a connection to make sure
contacts are set
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.5 A punchdown tool in action
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Software Tools
• Built-in tools
– tracert/traceroute
– ipconfig/ifconfig/ip
– arp, ping, arping, and pathping
– nslookup/dig
– hostname
– route
– nbtstat and netstat/ss
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Software Tools (cont’d.)
• Third-party tools
– Packet sniffer
– Port scanners
– Throughput testers
– Looking glass sites
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The tracert/traceroute
Commands
• Traces all of the routers between two points
• Assist in diagnosing where a problem lies
– For problems in reaching a remote system
• If traceroute stops at a certain router
– Problem is the next router or the connection
• Some routers block ICMP packets containing
traceroute information
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Tracing route to adsl-208-190-121-38.dsl.hstntx.swbell.net
[208.190.121.38] over a maximum of 30 hops:
1
1 ms
<1 ms
1 ms Router.totalhome
[192.168.4.1]
2
38 ms
41 ms
70 ms adsl-208-190-12138.dsl.hstntx.swbell.net [208.190.121.38]
Sample traceroute output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The ipconfig/ifconfig/ip
Commands
• Displays IP settings
• ipconfig without parameters
– Provides basic information only
• ipconfig /all
– Gives detailed information (e.g., DNS servers and
MAC addresses)
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Ethernet adapter Main:
Connection-specific DNS Suffix . :
IPv6 Address . . . . . . . . . . :
2001:470:bf88:1:fc2d:aeb2:99d2:e2b4
Temporary IPv6 Address . . . . . :
2001:470:bf88:1:5e4:c1ef:7b30:ddd6
Link-local IPv6 Address. . . . . : fe80::fc2d:aeb2:99d2:e2b4%8
IPv4 Address . . . . . . . . . . : 192.168.4.27
Subnet Mask . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . : fe80::223:4ff:fe8c:b720%8
192.168.4.1
Tunnel adapter Local Area Connection* 6:
Media State . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Sample ipconfig output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
options=3<RXCSUM,TXCSUM>
inet6 ::1 prefixlen 128
inet 127.0.0.1 netmask 0xff000000
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
nd6 options=1<PERFORMNUD>
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=10b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV>
ether 3c:07:54:7a:d4:d8
inet6 fe80::3e07:54ff:fe7a:d4d8%en0 prefixlen 64 scopeid 0x4
inet 192.168.4.78 netmask 0xffffff00 broadcast 192.168.4.255
inet6 2601:e::abcd:3e07:54ff:fe7a:d4d8 prefixlen 64 autoconf
inet6 2601:e::abcd:b84e:9fad:3add:c73b prefixlen 64 autoconf
temporary
nd6 options=1<PERFORMNUD>
media: autoselect (1000baseT <full-duplex,flow-control>)
status: active
Sample ifconfig output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state
UNKNOWN group default qlen 1000
link/ether 00:0c:29:e0:b2:85 brd ff:ff:ff:ff:ff:ff
inet 192.168.4.19/24 brd 192.168.4.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 2601:e:0:abcd:8cfb:6220:ec23:80a/64 scope global temporary dynamic
valid_lft 86221sec preferred_lft 14221sec
inet6 2601:e:0:abcd:20c:29ff:fee0:b285/64 scope global dynamic
valid_lft 86221sec preferred_lft 14221sec
inet6 fe80::20c:29ff:fee0:b285/64 scope link
valid_lft forever preferred_lft forever
Sample Linux’s ip output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The arp Command
• The ARP table records IP address–MAC
address pairs
• The arp command enables viewing and
changing the ARP table on a computer
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Interface: 192.168.4.57 ––– 0xc
Internet Address Physical Address Type
192.168.4.1 b8-9b-c9-7d-e7-76 dynamic
192.168.4.2 00-87-b6-7e-ae-23 dynamic
192.168.4.8 67-ab-cc-aa-fe-ed dynamic
192.168.4.12 23-b5-94-17-d7-33 dynamic
192.168.4.13 4b-4b-4c-4d-4e-46 dynamic
192.168.4.14 55-55-55-55-55-55 dynamic
Sample arp -a output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The ping Command
•
•
•
•
•
Queries by name or IP address
Uses ICMP packets
Works across routers
Problem: devices can block ICMP
Defaults to IPv4
– Use -6 switch for IPv6: ping -6 (Windows) or ping6
(Unix/Linux/OS X)
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Pinging 192.168.4.19 with 32 bytes
Reply from 192.168.4.19: bytes=32
Reply from 192.168.4.19: bytes=32
Reply from 192.168.4.19: bytes=32
Reply from 192.168.4.19: bytes=32
of data:
time<1ms
time<1ms
time<1ms
time<1ms
TTL=64
TTL=64
TTL=64
TTL=64
Ping statistics for 192.168.4.19:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Sample ping output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The arping Command
• Uses ARP frames rather than ICMP packets
• Does not work across routers
• Is supported by UNIX and UNIX-like systems
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
ARPING 192.168.4.27 from 192.168.4.19 eth0
Unicast reply from 192.168.4.27 [00:1D:60:DD:92:C6]
Unicast reply from 192.168.4.27 [00:1D:60:DD:92:C6]
Unicast reply from 192.168.4.27 [00:1D:60:DD:92:C6]
Unicast reply from 192.168.4.27 [00:1D:60:DD:92:C6]
Sample arping output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
0.875ms
0.897ms
0.924ms
0.977ms
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The pathping Command
• Combines the functions of ping and
tracert
– Also adds some additional functions
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Tracing route to xeroxpaser.totalhome [182.168.4.17]
Over a maximum 30 hops:
0 local-PC.totalhome [192.168.4.53]
1 xrxphsr.totalhome [192.168.4.17]
Computing statistics for 25 seconds...
Source to Here This Node/Link
Hop RTT
Lost/Sent – Pct
Lost/Sent – Pct Address
0
local-PC.totalhome [192.168.4.53]
0/ 100 – 0%
:
1 0ms
0/ 100 – 0%
0/ 100 – 0% xrxphsr.totalhome [192.168.4.17]
Trace complete
Sample pathping output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The nslookup/dig Commands
• Both diagnose DNS problems
• nslookup (all operating systems)
– Only choice by default on Windows systems
• dig (UNIX/Linux)
– Provides more verbose output by default than
nslookup
– Example of the dig command:
dig mx totalsem.com
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
; <<>> DIG 9.5.0-P2 <<>> mx totalsem.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6070
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1
;; QUESTION SECTION:
;totalsem.com.
IN
MX
;; ANSWER SECTION:
totalsem.com.
86400
IN
MX
10
mx1c1.megamailservers.com.
totalsem.com.
86400
IN
MX
100
mx2c1.megamailservers.com.
totalsem.com.
86400
IN
MX
110
mx3c1.megamailservers.com.
Output for the dig command
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The hostname Command
• Simplest of all utilities covered here
• Returns name of host from which it runs
• hostname sample output:
C:\>
C:\>hostname
mike-win8beta
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The mtr Command
• Dynamic (keeps running)
• Equivalent to traceroute
• Not supported by Windows
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
My traceroute [v0.73]
totaltest (0.0.0.0)
Keys: Help
Display mode
Restart statistics Order of fields quit
Packets
Pings
Host
Loss%
Snt Last
Avg Best Wrst StDev
1. Router.totalhome
0.0%
5
0.8
0.8
0.7
0.9
0.1
2. adsl-208-190-121-38.dsl.hstntx.s
0.0%
4 85.7 90.7 69.5 119.2 20.8
Sample mtr output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The route Command
• Used to display and edit the local system’s
routing table
• Type route print or netstat -r
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
===========================================================================
Interface List
8 ...00 1d 60 dd 92 c6 ...... Marvell 88E8056 PCI-E Ethernet Controller
1 ........................... Software Loopback Interface 1
===========================================================================
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination
Netmask
Gateway
Interface
Metric
0.0.0.0
0.0.0.0
192.168.4.1
192.168.4.27
10
127.0.0.0
255.0.0.0
On-link
127.0.0.1
306
127.0.0.1 255.255.255.255
On-link
127.0.0.1
306
127.255.255.255 255.255.255.255
On-link
127.0.0.1
306
169.254.0.0
255.255.0.0
On-link
192.168.4.27
286
169.254.214.185 255.255.255.255
On-link 169.254.214.185
276
169.254.255.255 255.255.255.255
On-link
192.168.4.27
266
192.168.4.0
255.255.255.0
On-link
192.168.4.27
266
192.168.4.27 255.255.255.255
On-link
192.168.4.27
266
192.168.4.255 255.255.255.255
On-link
192.168.4.27
266
224.0.0.0
240.0.0.0
On-link
127.0.0.1
306
224.0.0.0
240.0.0.0
On-link 169.254.214.185
276
224.0.0.0
240.0.0.0
On-link
192.168.4.27
266
255.255.255.255 255.255.255.255
On-link
127.0.0.1
306
255.255.255.255 255.255.255.255
On-link 169.254.214.185
276
255.255.255.255 255.255.255.255
On-link
192.168.4.27
266
===========================================================================
Persistent Routes:
None
Sample route print output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The nbtstat Command
• Windows-only program
• Command-line equivalent of Window’s My
Network Places or Network icon
• Always run with a switch
• nbtstat –n shows the local NetBIOS
names
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Main:
Node IpAddress: [192.168.4.27] Scope Id: []
NetBIOS Local Name Table
Name
Type
Status
--------------------------------------------MIKESPC
<00> UNIQUE
Registered
TOTALHOME
<00> GROUP
Registered
MIKESPC
<20> UNIQUE
Registered
TOTALHOME
<1E> GROUP
Registered
Sample nbtstat -n output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The netstat Command
• Comes with Windows systems
• Displays the current state of running IP
processes
• Shows what sessions are active
• Provides statistics based on ports or protocols
• Type netstat to show only current sessions
• Type netstat –r to show the routing
table; identical to route print
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Active Connections
Proto
Local Address
TCP
127.0.0.1:27015
TCP
127.0.0.1:51090
TCP
127.0.0.1:52500
TCP
192.168.4.27:54731
TCP
192.168.4.27:55080
TCP
192.168.4.27:56126
TCP
192.168.4.27:62727
TCP
192.168.4.27:63325
TCP
192.168.4.27:63968
Foreign Address
MikesPC:51090
MikesPC:27015
MikesPC:52501
72-165-61-141:27039
63-246-140-18:http
acd4129913:https
TOTALTEST:ssh
65.54.165.136:https
209.8.115.129:http
Sample netstat output
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
State
ESTABLISHED
ESTABLISHED
ESTABLISHED
CLOSE_WAIT
CLOSE_WAIT
ESTABLISHED
ESTABLISHED
TIME_WAIT
ESTABLISHED
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The ss Command
• Has eclipsed netstat on the Linux side
• Faster and more powerful than netstat
• Sample output (filtered to show only TCP
connections):
State
CLOSE-WAIT
CLOSE-WAIT
ESTAB
Recv-Q
28
28
0
Copyright © 2015 by McGraw-Hill Education. All rights reserved.
Send-Q
0
0
0
Local Address:Port
10.0.2.15:52161
10.0.2.15:46117
10.0.2.15:55542
Peer Address:Port
91.189.92.24:https
91.189.92.11:https
74.125.239.40:http
Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Packet Sniffer
• Also called protocol analyzer or packet
analyzer
• Intercepts and logs network packets
• Many choices
– Software or dedicated hardware devices
• Software example: Wireshark
• Command-line tool: tcpdump
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.6 Wireshark in action!
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Port Scanners
• Probe ports on another system
• Log the state of scanned ports
• Uses
– Find unintentionally open ports that could make a
system vulnerable to attack
– Hackers can use to break into systems
• Nmap is available for UNIX and Windows
• Angry IP Scanner is a Windows port scanner
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.7 Angry IP Scanner
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Throughput Testers
• Measure the data flow in a network
• The appropriate tool depends on the type of
network throughput you want to test
• Several speed-test sites are available for
checking an Internet connection’s throughput
– Example: MegaPath’s Speakeasy Speed Test
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.8 Speed Test results from Speakeasy
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Looking Glass Sites
• Browser-accessible remote servers
• Contain collections of diagnostic tools
– Also Border Gateway Protocol (BGP) query tools
• Allow selection of origination location, target
destination, and desired diagnostic
– May also test IP version
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Troubleshooting: Two Basic Rules
• Do no harm.
• Always make good backups!
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The Troubleshooting Process
1. Identify the problem
a.
b.
c.
d.
e.
f.
Gather information
Duplicate the problem, if possible
Question users
Identify symptoms
Determine if anything has changed
Approach multiple problems individually
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The Troubleshooting Process (cont’d.)
2. Establish a theory of probable cause
a. Question the obvious
b. Consider multiple approaches
i. Top-to-bottom/bottom-to-top OSI model
ii. Divide and conquer
3. Test theory to determine cause
a. If confirmed, determine next steps
b. If not confirmed, establish new theory or
escalate
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
The Troubleshooting Process (cont’d.)
4. Establish a plan of action to resolve the
problem and identify potential effects
5. Implement the solution or escalate as
necessary
6. Verify full system functionality
7. Implement preventative measures, if
applicable
8. Document findings, actions, and outcomes
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Identify the Problem
• The true problem may be different from what
the user tells you
– The symptom may indicate a larger problem
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Gather Information, Duplicate the
Problem, Question Users, and
Identify Symptoms
• Direct support
– Observe what is or is not happening
• Phone support
– Use closed-ended or open-ended questions
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Gather Information, Duplicate the
Problem, Question Users, and
Identify Symptoms (cont’d.)
• Duplicate the problem
– Can rule out user error as the cause
• Determine the scope of the problem
– Are other users affected?
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Determine If Anything Has Changed
• Example questions to users
– “What was happening when the problem
occurred?”
– “Has anything changed on the system recently?”
– “Has the system been moved recently?”
• Check your documentation
– Includes system and hardware logs
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Approach Multiple
Problems Individually
• Break down complicated problems into
manageable pieces
– Methodically tackle each sub-problem
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Establish a Theory of Probable Cause
• Based on experience
• Select the most probable cause from a list of
all possible causes
• Question the obvious
– Example: check to ensure a printer is powered on
• Considering multiple approaches can help
avoid becoming locked in to a single approach
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Example Scenario
• User cannot access the server
– Opens database program and clicks on a recent
document
– The document does not open
• Consider a top-to-bottom or bottom-to-top
OSI model approach
– Application layer: consider whether a problem
with the API is the possible cause
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
OSI Model Approach
• Presentation layer: consider encryption
between the application and database server
• Session layer: consider whether a port on the
server is blocked
• Transport layer: consider the effect of extreme
traffic
• Network layer: consider whether the IP
address of the database server has changed
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
OSI Model Approach (cont’d.)
• Data link layer: consider whether the MAC
address of the user’s machine or the database
server is blacklisted
• Physical layer: check for a disconnected cable
or a bad NIC
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Other Options for Tackling
Multiple Options
• Reverse the approach using the OSI model
beginning with Layers 1 and 2
• Divide and conquer approach
– Choose the OSI layer starting point based on a
general sense of where the problem lies
– If this starting layer is not the problem, move up
or down the OSI model with new theories of
probable cause
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Test the Theory to Determine Cause
• Test the theory without changing anything
– You may not have permission to make the fix
– The fix may have unanticipated repercussions
• Escalate the problem
– Inform other parties for guidance
– Pass the job to another authority with control
over the device or issue
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Establish a Plan of Action and
Identify Potential Effects
• Write down the steps of your action plan
• Identify the potential effects of the actions
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Implement the Solution or
Escalate as Necessary
• Examples of implementation
– Give advice to a user over the phone
– Install a replacement part
– Add a software patch
• Try only one solution at a time
• Document what you do
• Test the solution
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Verify Full System Functionality and
Implement Preventative Measures
• In the example scenario:
– Have the user open the database while you are
still at the computer
• Implement preventative measures to avoid a
repeat of the problem
– May involve educating the user to do or not do
something
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Document Findings, Actions,
and Outcomes
• Purpose of documentation
– Creates a knowledge database for future
reference
• Eliminates duplication of work
– Allows tracking problem trends
• Anticipate future workloads
• May identify less reliable brands or models
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Resolving Common Network Issues
• Basic categories of network issues
– Fixing problems at a user’s workstation, work
area, or a server
– Connecting to resources on the LAN
• Or WAN, but these may need to be escalated
• Stumbling block
– Observed symptom may be the same for a variety
of root causes
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
“We Can’t Access Our
Web Server in Istanbul!”
• Example scenario
– Everyone has local and Internet Web site access
– The firewall configuration was recently changed
• Technician’s theories
– The remote server is down
– The remote site is inaccessible
– The local firewall is preventing communication
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
“We Can’t Access Our
Web Server in Istanbul!” (cont’d.)
• Quickest to test approach
– Confirm all local office workstations cannot reach
the remote server
• Using different hosts, try ping and ping6 utilities
• Traceroute shows functional path to the router, but
no server response
• Run the mtr utility from a Linux box and
simultaneously run pathping on a Windows system
– Call the remote site to ask about the status
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
“We Can’t Access Our
Web Server in Istanbul!” (cont’d.)
• Determine whether the site is reachable from
outside the local office
– Would confirm or eliminate the theory of a local
firewall configuration issue
– Using a looking glass site tool, perform a ping test
• No pings receive a response from the server in question
• Other servers at the remote site do respond
• Conclusion: the remote server is down
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
“We Can’t Access Our
Web Server in Istanbul!” (cont’d.)
• Possible causes of an unresponsive server
– Local power outage or blown circuit breaker
– Failed NIC on the server
– Network cable disconnected
– Improper network configuration on the server
– A changed patch cable location in the rack
– Failed component in the server
– Server shutdown
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Hands-On Problems
• Problems you can fix at the workstation, work
area, or server
– Include physical and configuration problems
• Possible causes of an unreachable network
device
– Power failure or power anomalies
– Hardware failure
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Possible Causes of
Unreachable Network Device
• EMI or RFI disrupting signals on copper cable
• Interface errors, such as improper wall jack
installation
• Incorrect termination
• Cable to the workstation might be bad
– Crossover versus straight-through cable
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Other Types of Hands-On Problems
• Incorrect IP configuration
• Incorrect default gateway IP address
information
• Out of date DNS entry
• Simultaneous wireless/wired connections
– Can create a network failure
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.9 TCP/IP settings in Windows Server
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.10 Network Connections Advanced Settings
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
LAN Problems
• Issues can arise when duplicating machines
and using static IP addresses
– A solution is to change the IP address on the new
machine to an unused static IP or to DHCP
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Server Misconfigurations
• Misconfigured DHCP settings
– On a host: effect is limited to that host
– On a DHCP server: many more machines affected
• Misconfigured DNS settings
– On a DNS server: the server might direct hosts to
incorrect sites or no sites at all
– On a client: name resolution stops and the user
experiences a downed network
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Server Misconfigurations (cont’d.)
• Clues to server misconfiguration
– Success in pinging a file server by address but not
by name ► points to a DNS issue
– Failure to discover neighboring devices/nodes ►
may point to DHCP or DNS misconfiguration
• To fix misconfiguration
– Go into the network configuration for the client or
the server and look for problems
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Adding VLANs
• Example: Figure 21.11
– Bill intended to assign 6 ports to each VLAN
• Instead assigned 7 ports to VLAN 1 and 5 ports to
VLAN 2
– Result: interface misconfiguration producing an
incorrect VLAN assignment
• Patch cable placement errors can also occur
– Keep proper records of patch cable assignments
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.11 Bill’s VLAN assignments
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Link Aggregation Problems
• Scaling an Ethernet network
– Bandwidth issues: one choice is to upgrade the
switch and server NIC to the next higher Ethernet
standard
– Other approach: link aggregation or NIC teaming
• Protocols
– Link Aggregation Control Protocol (LACP)
– Cisco’s Port Aggregation Protocol (PAgP)
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Link Aggregation Problems (cont’d.)
• Enabling LACP
– Need two or more interconnected network
interfaces configured for LACP
– Devices communicate over multiple ports at the
same speeds and form a single logical port
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.12 LACP
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Link Aggregation Problems (cont’d.)
• Active ports
– Default to using LACP, if possible
• Passive ports
– Wait for active ports to initiate conversation
• Common network error with LACP setups
– Both sides of the connection set to passive ► an
example of NIC teaming misconfiguration
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
WAN Problems
• Possible origination points
– Local machines
– LAN switches
– Routers that interconnect the WAN
– Switches within the distant network
– Distant machines
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Router Problems
• Nonfunctioning connections between
networks
• Potential causes
– Physical problems with the router or router
interface modules
– Loss of power
– Problems with the Access Control List
• A misconfiguration can lead to missing IP routes
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Router Problems (cont’d.)
• Key tool for determining a router problem
beyond the local LAN is traceroute
• Run traceroute to the default gateway
– A failed traceroute indicates a local issue
– If the traceroute comes back positive, run it to
an Internet site
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Figure 21.13 Good connection
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
ISPs and MTUs
• MTU mismatch
– A network’s packets are so large that they must be
fragmented to fit into the ISP’s packets
• Ways to remedy MTU mismatch
– Techs can tweak the MTU settings to improve
throughput
– Path MTU Discovery (PMTU) determines the best
MTU setting automatically
• PMTU runs under ICMP
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Appliance Problems
• Network appliances
– Complex boxes with multiple features, e.g.,
routing, NAT, switching, IDS, firewall, and more
• NAT rules take precedence over appliance’s
routing table entries
– Tech must set the NAT rule order correctly
• Fix a NAT interface misconfiguration by setting
up the network appliance correctly
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Company Security Policy
• Example
– Amount of traffic between two company locations
on a dedicated connection is causing huge
bandwidth issues
– Cause: sales department is sending large video
files
– Need to limit bandwidth for a single department
– Need a blocking policy to enforce the limit
– Need a company fair use policy to state limits
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Beyond Local—Escalate
• Problems to escalate
– Problem that exceeds the tech’s skill level
– Problem that involves third-party equipment
• Large organizations typically have clear
escalation procedures
• CompTIA-recognized escalation situations
– Broadcast storms, switching loops, routing
problems, routing loops, and proxy ARP
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Broadcast Storms
• Result of one or more devices sending
nonstop broadcast frames onto the network
• Symptom: every computer on the broadcast
domain cannot connect to network
– Many users contact you simultaneously
– Call a supervisor to get support
• Isolate devices until the problem device is
identified
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Switching Loops
• Multiple switches inadvertently connected
together to form a loop
– Also called bridging loops
• Symptoms are identical to a broadcast storm
• Rarely take place on a well-running network
– Most switches use the Spanning Tree Protocol
• Escalate the problem to discover the person
making switch changes
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Routing Loops
• Occur when interconnected routers loop
traffic
• Cause routers to respond slowly or not at all
• Clue: a huge amount of traffic—far more than
the usual traffic—on the links between the
routers
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Proxy ARP
• Process of making remotely connected
computers act as if they are on local LAN
– VPN is the classic example
• Almost all proxy ARP problems involve the
VPN concentrator
– With misconfigured proxy ARP settings, the VPN
concentrator sends what looks like a DoS attack on
the LAN
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
End-to-End Connectivity
• End-to-end principle
– Applications and work should happen only at
endpoints in a network
– The Internet was founded on this principle
• Modern networks
– Much activity happens at intermediate devices
reflects the current state of the Internet
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
End-to-End Connectivity (cont’d.)
• Connecting users with essential resources
within a smaller network, e.g., aLAN or a
private WAN
– A tech’s job includes ensuring connections happen
fully
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Troubleshooting Is Fun!
• Apply good troubleshooting methodology
• Constantly increase your network knowledge
• Become a troubleshooting artist
– Benefit: Easier to deal with a network disaster
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Internet of Things (IoT)
• Everyday objects capable of communicating
with each other
• A large-scale idea
– Changes promised by IoT
– Challenges involved with IoT
– Technologies making IoT possible
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Utopia of things
• Vast array of real-world smart objects
– Collecting sensor data
– Communicating that data with other objects or
computers
– Making decisions based on it
• Examples of smart objects in the home and at
the gym
• Larger scale applications, e.g., the power grid
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Challenges?
•
•
•
•
•
•
•
Bugs
Hacking
Feedback loops
Interoperability
Waste and obsolescence
Power
Scale
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Supporting Technologies
• IPv6 is essential for enough address space
• Ultra-low-power wireless radios
– Communications with vast networks of sensors
• IEEE 1905.1 is a hybrid networking standard
– MoCA provides Ethernet access through existing
coaxial cabling
– HomePlug (IEEE 1901) provides high-speed home
networking—Ethernet over power (EoP)
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Supporting Technologies (cont’d.)
• nVoy: the branding for IEEE 1905.1
• nVoy-certified networking equipment:
– Will create a single network for devices spread
across Ethernet, Wi-Fi, MoCA, and HomePlug
connections
– Can make intelligent decisions when
communicating with devices that use more than
one connection type
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Mike Meyers’ CompTIA Network+® Guide to Managing and
Troubleshooting Networks, Fourth Edition (Exam N10-006)
Looking Ahead
• There is no clear finish line to mark the arrival
of IoT
– The networking requirements leading to IoT will
be incremental
• One day communications between smart
devices may overtake the traffic generated by
human users
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