Connecting Two Routed/Switched Networks

[doverfpl]

In the small public library where I work, we have two physical networks of different types running in our building. One network connects to the County, via Verizon frame relay, and uses a 56k DSU connected to a Cisco 2507 router and a Linksys 24-port 10/100 switch. The other network connects to the Internet via Cablevision, and uses a cable modem connected to a Linksys BEFSR81 router with built-in 8 port switch.

My question is this: I need the computers on both networks to see each another, but I cannot change any of the settings on the Cisco router or any of the PC's on that network (IP addresses or subnet masks; the County doesn't allow it). So, how do I get these machines talking to one another?

The two networks do not need to share Internet connections (in fact, under the County's rules, they can't!). I just need the machines to be able to share files.

My intuition tells me that I probably only need to install a crossover cable between the two switches. Am I right? Is this the way to go? Or, am I just asking for trouble by trying to do it this way?

 

[svermill]

You would in all likelyhood need to introduce a router in between the networks. You can't simply connect the two networks together at layer 2 (e.g. a crossover cable between switches) and expect the machines to be able to talk to one another. As they are in different networks, a router is necessary to move traffic. Also, I think you are going to have trouble if you can't change any of the configs on the existing equipment. Finally, I would ask how you plan to exchange information between the machines on the two networks. I don't know a lot about designing Windows networks, but I do know that there is a level of complexity involved. Machines on differnt networks have to become aware of one another, just for example. You may need to bring in a consultant or do some heavy homework to figure out what your options are. You might find someone willing to help you on a volunteer basis. Especially given the number of folks out of work in IT these days. Just some thoughts....

 

[doverfpl]

I hadn't thought of the possibility of adding a third router to move traffic betweent the two existing networks. It sounds right, but I just don't know.

Basically, as I said above, each network already has its own Internet connection, and I do not need to share these connections. I only need to move files between machines on one network's LAN to machines on the other network's LAN. Each LAN is configured using Microsoft Networking, available in the Windows OS.

The machines on each LAN have static IP addresses and share the same subnet mask, though the subnets are different on the two networks. Each machine on both networks has its own ID, and they all share the same workgroup designation.

My big fear is that the two Internet connections are going to end up fighting with one another, leading to irresolvable conflicts that I am not prepared to deal with (though I am quite proficient inside a PC box - upgrades, repairs, tweaks, etc. - I am a networking novice, at best).

I have designed and installed a number of Windows networks (LANs) with what I would call a "traditional" configuration: DSU>router>switch>PCs. I have never had any trouble understanding the necessities of this type of setup. My confusion arises from the fact that I have never tried to tie two of these networks together before.

Thanks, svermill. Any further help is appreciated.

 

[svermill]

Sounds like you have the expertise necessary to pull this off. Think about what a Windows machine does when it wants to reach a PC in the same subnet. It ARPs out for the MAC address. When it wants to reach a destination outside of it's subnet, it ARPs for its default gateway. If you don't add that third router, you could enter static routes into each PC. But that seems like it could get ugly pretty quickly. Also, I think you'd need the capability to add secondary addresses on each routers ethernet interface. I'm told that Cisco is discontinuing support for secondary addresses in upcoming IOS releases. Don't know what brand of routers you have now?

In general, if it's in your budget, you need to route between subnets. Overlapping layer 2 networks are a mess and not recommended (hence Cisco discontinuing support). And remember that you could possibly use a PC to route between subets if the traffic load isn't high (I've never done this, but I know this is done in small networks such as yours).

As for your internet connectivity, you certainly could have some issues to deal with. I don't think it would be anything really serious. Here again though, I don't see how you're going to do this without being able to change any configs. If you introduce a third router, but keep all host configs the same (e.g. keep their original default gateways), your routers have to become aware of the other subnet so they can forward traffic appropriately (and/or send an appropriate ICMP redirect). You would enable this through static routes or a routing protocol - on all three routers.

Now I mostly deal with large networks, so I'm no expert in small LAN design. Somebody might have better advice. But I do know that overlapping layer 2 networks are a headache to be avoided if possible.

Best of luck to you and please let us know what you settle on and how it goes for you.