Wireshark is your go-to network troubleshooting tool. Using it, you can quickly identify problem services, protocols, and ports and even detect network attacks. You can also use Wireshark to baseline your network and recommend upgrades. It's free to download and use.
Wireshark is tested in the WCNA exam but packet analysis is also tested in most IT security exams as well as advanced Cisco certifications.
If you want to learn more, you can join this website or our labs-only sister website 101labs.net.