Software under linux, to view the network load?

Question

System administration

Recommend software for network testing and network traffic analysis.
A network - multilevel, about 5 vlan. Network projectors are also used (about 10).
On some vlans, packets have large delays, especially on a subnet that is not in AD.
I want to monitor all networks from under linux and draw conclusions, where to climb, what to watch?
There are routers on each floor and configured to forward.
About 400 PCs + peripherals (MFPs, projectors) are also networked.

Answer 1

• iftopfor an initial rough estimate
  
• tcpdumpfor detailed sniffing
  

Answer 2

Offhand, I can offer zabbix. You can set it up for a long and dreary time, but in the end you will get what you want and how you want. (I won’t tell you about testing whether it’s possible or not)
An easier option is to try putting a piece of hardware like Mikrotik in a key place in the network (but this can create a very bottleneck) and watch traffic through it, etc. From it, you can even load a certain Ip with packets of different sizes and intervals, and much more.