租户网络与openstack中现有网络的通信

I am trying to make the tenant network communicate with an existing network which did not build in OpenStack. we all know we can allocate a floating IP to VM with external network, but now we have no need access Internet, so we would not allocate IP to VM, just want to make tenant VM can communicate with another network, how do I map the existing network to OpenStack and create router between them? Any help would be appreciated.Thanks!I am trying to make the tenant network communic