We can get basic network informations (to start an analysis) with the following commande:

tcpdump -nntttte -i <interface> -A -s0
The option:
-nn: unactivates the name and port resolution
-tttt: displays the date and the hour in a readable format
-e: adds the MAC address
-i: selects the network interface
-A: displays the data in ASCII format
-s0: forces the display of all the contents (otherwise there could be some truncatures)