Performing Troubleshooting Procedures

During and after configuring your devices on a network, it is best practice to verify the IP addresses on each device.

Table 2.2 shows the IP address scheme for the lab topology:

Table 2.2: IP addressing scheme

Up next, you will learn about various troubleshooting commands for both end devices and Cisco devices.

Verifying IP Configurations on End Devices

The following commands are commonly used by network professionals in their troubleshooting process to identify whether they’ve assigned the appropriate IP address on a host device:

• ipconfig: A Windows-based command for displaying the IP address, subnet mask, and default gateway addresses of each interface
• ipconfig /all: A Windows-based command that displays additional addressing such as the media access control (MAC) address and DNS server addresses on each interface
• ifconfig: A Linux-based command for displaying the IP addresses and interfaces
• ip address: A Linux-based command for displaying IP addresses and interfaces on Linux systems

Verifying IP Configurations on Cisco Devices

The following are useful Cisco IOS commands for verifying IP configurations on Cisco devices:

• show ip interface brief: Displays a summary of all interfaces, their status, and assigned IP addresses
• show ip interface [interface-ID]: Shows IP-related information for a specific interface
• show interfaces [interface-ID]: Shows detailed information about a specific interface
• show running-config: Displays the device’s current configurations, including the IP addresses that are configured on each interface

Verifying End-to-End Connectivity

The ping and traceroute utilities are software-based tools that are integrated into many operating systems, such as Windows, Linux, macOS, and even Cisco IOS. The ping utility leverages Internet Control Message Protocol (ICMP) for checking end-to-end connectivity between a sender and a destination host. ICMP is a Layer 3 network protocol that is integrated within the OSI and TCP/IP network model that sends error and operational messages to identify whether there is a failure or a successful connection between devices over a network. Additionally, the traceroute utility also leverages ICMP for checking each hop along the path from a sender to a destination host.

Figure 2.47 shows how to use the ping utility on a Windows-based system:

Figure 2.47: ping results on a Windows device

As shown in the preceding screenshot, the PC sent four ping requests to 172.16.1.10 (server) and got four ping replies, which indicates the destination host is online and has end-to-end connectivity.

Note

The ping [destination host] command is applicable to Windows, Linux, macOS, and Cisco IOS devices.

Figure 2.48 shows the ping output on a Cisco IOS router:

Figure 2.48: ping results on a Cisco device

Cisco IOS does not provide a similar output to Windows or Linux operating systems – rather, it provides various symbols with different meanings, such as the following:

• !: Successful – ping requests are sent and received.
• .: Request timeout – the ping request is sent to the destination but no response is received. This is caused by a return path being missing from the destination host or ICMP responses being disabled on the recipient device.
• U: Destination unreachable – the sender or default gateway does not have a route to the destination host or network. This is caused by a missing route from the routing table of the sender or default gateway.

Additional Troubleshooting Tips

The following are common troubleshooting tips:

• Verify the destination device is powered on
• Verify the cables are connected to the correct interfaces
• Ensure the IP addresses are configured on the correct interfaces
• Ensure the interfaces are administratively up

Having completed this section, you have learned about the fundamental checks to perform. As you progress further in this book, you will learn about more technologies and their troubleshooting procedures.