I'm sorry, I don't fully understand what you've written. I'll try to answer anyway...
A VPN, as you are aware, is basically an interconnection between two (or more) subnets, in a server-client basis. Of course, the client will always need to initiate the connection...
The fact that you need broadcasting doesn't make it impossible; you have to say which one acts as a server/client, and then push routes. For the broadcast, you'll need a wins server (or similar). Check this
how to, maybe it will help.
Regarding the keep alive, I've never monitored the quantity of information nor know how much it reduces the performance. Nevertheless, in OpenVPN, you can define it in time and retries. In the same site I posted here above, there is a study about performance, that I quote to you:
As you might expect, encryption takes its toll on network performance. But, in practice, network throughput will be limited more by the Internet connections of both the OpenVPN server and client, than by OpenVPN itself. For my setup, I get speeds of around 35 kbps, but the client side of my network uses a wireless point-to-point Internet provider that sounds good on paper, but in reality, is horribly unreliable.
To get a better look at OpenVPN's true performance, I set up both the server and client locally connected through a gigabit switch and transfered some files through them over SMB. Direct transfer (without OpenVPN) clocked in at around 38 Mbps. The same transfer over an encrypted tunnel was barely able to top 4 Mbps. But again, unless you have top-tier fiber-based connections on both ends of your encrypted tunnel, you're unlikely to be limited by OpenVPN itself.
So you definitely pay the price on the performance side, but you gain the ability to securely transfer data over insecure connections.
If you don't want to dedicate a computer at each end of an OpenVPN tunnel, there are implementations of OpenVPN that run on the limited hardware of consumer grade routers (like the Linksys WRT54G) through DD-WRT or OpenWrt. So don't worry about old hardware slowing you down!
Translation: If you are willing to sacrifice security in exchange for performance... it will become as fast as your ISP allows it. I don't recommend it at all. If you need a VPN connection that bad (without losing performance) the best would be to rent a VPN service (with dedicated modems for it) or more, a dedicated line. Performance would still be a issue: depends of your budget.
By experience, I can guarantee that in bridge mode it suffers a lot: when i'm in laziness mode
I tend to administrate my server with NX. Inside enterprise's network (as the PC in this post
), in bridge mode it took sometime to connect; in route mode, it's instantaneous.