Regardless of whether the fault is present on a wireless or wired network, one of the first steps in a bottom-up strategy of troubleshooting should be to examine the LEDs, which indicate the current state or activity of a piece of equipment or connection. LEDs may change color or flash to convey information. The exact configuration and meaning of LEDs varies between manufacturers and devices. Figure 20-5 shows a typical wireless router with LEDs indicating power, system, WLAN, wired ports, and Internet (labeled WAN), USB, and Quick Security Setup (QSS, also known as Wi-Fi Protected Setup [WPS]).
Figure 20-5 LED Lights on a Wireless Router
Note
WPS (or QSS) has known vulnerabilities that allow a threat actor to gain access to your network. Therefore, a security best practice is to disable this feature. Refer to documentation to learn how to disable WPS or QSS.
On some devices, a single LED may convey multiple pieces of information, depending on the current status of the device. It is important to check the equipment documentation for the exact meaning of all indicators, but some commonality does exist.
Most devices have activity LEDs, which are often called link lights. A normal condition is for these LEDs to flash, indicating that traffic is flowing through the port. A solid green light typically indicates that a device is plugged into the port, but no traffic is flowing. No light typically indicates one or more of the following:
- Nothing is plugged into the port.
- There is an issue with the wired or wireless connection.
- A device or port has failed.
- There is a cabling issue.
- The wireless router is improperly configured; for example, a port was administratively shut down.
- The wireless router has a hardware fault.
- The device does not have power.
Whether the network is wired or wireless, you should verify that the device and ports are up and functional before spending large amounts of time trying to troubleshoot other issues.
Cabling Problems (20.2.3)
If the wired client is unable to connect to the wireless router, one of the first things to check is the physical connectivity and cabling. Cabling is the central nervous system of wired networks and one of the most common issues when experiencing inactivity.
There are several issues to watch for when cabling:
- Be sure to use the correct type of cable. Two types of UTP cables are commonly encountered in networking: straight-through cables and crossover cables. Using the wrong type of cable may prevent connectivity.
- Improper cable termination is one of the main problems encountered in networks. To avoid this problem, you should terminate cables according to standards. Terminate all cables on the same network via the T568A or the T568B termination standard, not both. Avoid untwisting too much of the wire pairs during termination. Crimp connectors on the cable jacket to provide strain relief.
- Maximum cable run lengths exist based on characteristics of the different cables. Exceeding these run lengths can have a serious negative impact on network performance.
- If connectivity is a problem, verify that the correct ports are being used between the networking devices.
- Protect cables and connectors from physical damage. Support cables to prevent strain on connectors and run cable through areas that will not be in the way.
Troubleshooting Commands (20.3)
A number of software utility programs can help identify network problems.
Overview of Troubleshooting Commands (20.3.1)
Most of these utilities are provided by the operating system as CLI commands. The syntax for the commands may vary between operating systems.
Some of the available utilities include
- ipconfig—Displays IP configuration information
- ping—Tests connections to other IP hosts
- netstat—Displays network connections
- tracert—Displays the route taken to the destination
- nslookup—Directly queries the name server for information on a destination domain
The ipconfig Command (20.3.2)
When a device does not get an IP address or has an incorrect IP configuration, it cannot communicate on the network or access the Internet. On Windows devices, you can view the IP configuration information with the ipconfig command at the command prompt. The ipconfig command has several helpful options, including /all, /release, and /renew.
The ipconfig command, shown in Example 20-1, is used to display the current IP configuration information for a host. Issuing this command from the command prompt displays the basic configuration information, including IP address, subnet mask, and default gateway.
Example 20-1 The ipconfig Command
C:\>
ipconfig
Windows IP Configuration
Ethernet adapter Ethernet:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Wireless LAN adapter Wi-Fi:
Connection-specific DNS Suffix . : lan
Link-local IPv6 Address . . . . . : fe80::a1cc:4239:d3ab:2675%6
IPv4 Address. . . . . . . . . . . : 10.10.10.130
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.10.10.1
C:\>
The ipconfig /all command, shown in Example 20-2, displays additional information, including the MAC address, IP addresses of the default gateway, and the DNS servers. It also indicates whether DHCP is enabled, the DHCP server address, and lease information.
Example 20-2 The ipconfig /all Command
C:\>
ipconfig/all
Windows IP Configuration
Host Name . . . . . . . . . . . . : your-a9270112e3
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : lan
Ethernet adapter Ethernet:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Realtek PCIe GBE Family Controller
Physical Address. . . . . . . . . : 00-16-D4-02-5A-EC
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Wireless LAN adapter Wi-Fi:
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Intel(R) Dual Band Wireless-AC 3165
Physical Address. . . . . . . . . : 00-13-02-47-8C-6A
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::a1cc:4239:d3ab:2675%6(Preferred)
IPv4 Address. . . . . . . . . . . : 10.10.10.130(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : Wednesday, September 2, 2020 10:03:43 PM
Lease Expires . . . . . . . . . . : Friday, September 11, 2020 10:23:36 AM
Default Gateway . . . . . . . . . : 10.10.10.1
DHCP Server . . . . . . . . . . . : 10.10.10.1
DHCPv6 IAID . . . . . . . . . . . : 98604135
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-1E-21-A5-84-44-A8-42-FC-0D-6F
DNS Servers . . . . . . . . . . . : 10.10.10.1
NetBIOS over Tcpip. . . . . . . . : Enabled
C:\>
How can this utility assist in the troubleshooting process? Without an appropriate IP configuration, a host cannot participate in communications on a network. If the host does not know the location of the DNS servers, it cannot translate names into IP addresses.
If IP addressing information is assigned dynamically, the ipconfig /release command, shown in Example 20-3, releases the current DHCP bindings. The ipconfig /renew command requests fresh configuration information from the DHCP server. A host may contain faulty or outdated IP configuration information, and a simple renewal of this information is all that is required to regain connectivity.