Diagnostic Commands

There are a variety of Cisco IOS and networking tools you can use for information gather and diagnostics. I've broken them up into a few groups to make them easier to remember, but there will often be overlap.

Connected Device Information

If you want to learn anything about a device, the show command is what you need. There are many sub-commands that you can use to reveal information about the device, but here are a few basic ones you should be familiar with.

`show running-config` and `show startup-config`

Can be used to compare the entire config between devices (or to check for unsaved changes)
Often a great place to start to gather basic config information.

`show interfaces`

Shows detailed Layer 1 and Layer 2 information about all interfaces or a specific interface
Allows use of the brief sub-command to summarize information

`show ip interface`

Shows detailed Layer 3 information about all interfaces or a specific interface
Can be summarized with the brief command
Can return IPv6 information with show ipv6 interface

`show ip route`

Shows the Routing Table, which detailed network routes and how they were learned

`show cdp neighbors` and `show lldp neighbors`

CDP is enabled by default on Cisco devices, and communicates identifying information between devices
This can be very helpful if you don't know which interface a Switch is plugged into, and can help you reconstruct the network topology

`show ip protocols`

Shows the running Dynamic Routing Protocols on a device, and their configurations

`show arp`

Shows the ARP entry (i.e., IP address and MAC Address) for all interfaces

Network Connectivity

Ping

ping [destination IP] sends ICMP echo requests to a destination IP address
A quick way to test two-way Layer 3 connectivity between hosts
1. The ICMP echo request must be able to reach the destination, and the ICMP echo reply must be able to return

Traceroute

A step further than Ping, traceroute [destination IP] sends UDP datagrams with incrementing TTL values to a destination IP, with a default target UDP port 33434
Each hop replies, and the route to the destination IP is mapped out.

Telnet

While Telnet is not secure for data transmission, it can be used to test Layer 4 connectivity between devices and verify open ports with just telnet [host name or IP] [port number]
1. Requires almost no setup, and the receiving device doesn't need to support it
2. You will receive a message stating whether the message was successful or nor

Debug and Logging Information

`debug [opt]``

The debug command shows real-time logging information directly in the console
1. undebug [opt] and undebug all (or shortened to u all) can stem the flow of information

Host Network Information

Below are sample output commands for the ping and ipconfig/ifconfig for different host operating systems.

Where applicable, I changed the code syntax highlighter to make the output more readable. Nothing really helped with ifconfig.

Ping Command:

Windows:
- Command: Open Command Prompt and use the "ping" command.
- Example: ping google.com
- Output:
Linux (using Terminal):
- Command: Open the terminal and use the "ping" command.
- Example: ping google.com
- Output:
macOS (using Terminal):
- Command: Open the terminal and use the "ping" command.
- Example: ping google.com
- Output:

ipconfig/ifconfig Command:

Windows:
- Command: Open Command Prompt and use the ipconfig command.
- Example: ipconfig
- Output:
Linux (using Terminal):
- Command: Open the terminal and use the "ifconfig" command.
- Example: ifconfig
- Output:
macOS (using Terminal):
- Command: Open the terminal and use the "ifconfig" command.
- Example: ifconfig
- Output: