I setup a centos machine running kvm & cloudmin for hosting a virtual router and a bunch of virtual linux machines.
The normal ethernet interface sets up as a bridge (br0) which is the uplink for the hypervisor and MT router VM.
Add a second bridge interface (br1) to the kvm hypervisor. The second bridge interface is the "internal network" for the virtual routerOS machine and the interface of the subsequent virtual linux machines behind the router.
I even setup a VPN server on the virtual router for remote access to the internal servers.
Replaces a router, switch, and multiple servers with one neat little server.