It depends a bit on what you want to learn exactly. E.g. do you want to just learn about basic routing, BGP, MPLS, certain vendor specific stuff (like Cisco) et

It depends a bit on what you want to learn exactly. E.g. do you want to just learn about basic routing, BGP, MPLS, certain vendor specific stuff (like Cisco) etc. If you mostly just want to try out what you can archive on Linux you can get quite far with doing things via network namespaces (+whatever software you want to use, e.g. BIRD or Quagga) these days. Large pro on doing things via namespaces is that you can set them up & tear them down a lot quicker compared to VMs or physical hardware & scripting those steps is a lot easier. VMs & physical hardware provide more ways to do things. At least in the past there were various limitations on what you could do emulate with Cisco VMs for example (especially on some more advanced Nexus features).