1. PING- to check network connectivity:Ping is one of the widely used Linux network commands in network troubleshooting. It checks the availability of the network. It works by sending an ICMP echo request to check the connectivity of the network. Until you interrupt, the ping command will keep executing. However, you can restrict the number of echo requests.
2. tcpdump- packet analyzer:it is considered the most powerful command line tool which is used to capture or filter TCP /IP and other packets that are being received or sent over an IP Network on a particular interface of the switch or router. This access to the packets can be of immense importance when troubleshooting the network. You can capture packet from the specific port.
The other option given by it is that it enables the user to save captured packets to a file that can be used in future for analysis. The file is saved in pcap format, that can be accessed by tcpdump command. Another open source GUI based most powerful tool called Wire shark which also reads tcpdump pcap format files.