Simple routing

Vagrantfile

Vagrant.configure(2) do |config|
 config.vm.box = "minimal/xenial64"
 config.vm.synced_folder ".", "/vagrant", disabled: true

 config.vm.define :nodeL do |node|
  node.vm.network :private_network, ip: "192.168.30.2", virtualbox__intnet: "L"
  node.vm.provision :shell, inline: <<-SHELL
   ip route replace default via 192.168.30.3
  SHELL
 end

 config.vm.define :nodeR do |node|
  node.vm.network :private_network, ip: "192.168.40.2", virtualbox__intnet: "R"
  node.vm.provision :shell, inline: <<-SHELL
   ip route replace default via 192.168.40.3
  SHELL
 end

 config.vm.define :nodeX do |node|
  node.vm.network :private_network, ip: "192.168.30.3", virtualbox__intnet: "L"
  node.vm.network :private_network, ip: "192.168.40.3", virtualbox__intnet: "R"
  node.vm.provision :shell, inline: <<-SHELL
   sysctl net.ipv4.ip_forward=1
  SHELL
 end
end