Problem with multiple routers on one switch/subnet


While I am comfortable with basic network configuration, I came across an unusual network topology at work today that we are having a problem with and I could use some help figuring it out. Here is a basic description of how things are currently set-up:

At the core, there is a 24 port Netgear switch (JFS524). Connected to this switch is one PC with a statically assigned IP of, also connected to the switch are 6 Linksys routers (BEFSR41), assigned ip's Each router serves as a local DHCP server but has it's number of DHCP-assignable addresses restricted to 1, and each has it's DHCP starting address as respectfully. The routers are connected to the switch via one of the LAN ports on the router. Each router is then connected (also via one of its LAN ports) to a network enabled device (a product under test which is controlled via ethernet). Thus there is one router per test station. These devices (those under test) are removed from the network when the testing is completed and a new device is put in its place.

In theory, what we are trying to achieve is that when a device under test is connected, it will request an IP address and the router for that test station will always assign the same IP to devices connected at that station. The user at the PC will then be able to selectively connect to devices under test (through an application which is trivial for the purpose of this question) because the operator will always know which IP is associated with devices at a particular test stand.

This set-up is working most of the time, but we are experiencing an intermittent problem that is causing a lot of trouble. Occasionally when we connect a device to one of the routers, one of the other routers in the system ends up assigning the device its "assignable IP" instead of the router that the device is directly connected to. As you can see this kind of defeats the entire purpose of the setup and knowing which IP is assigned to a particular device based on the test stand it is at. Someone in my office suggested that connecting the routers to the switch via their WAN ports may help, but I'm worried this would lead to a slew of other addressing problems that would have to be worked out.

If anyone has any thoughts on how best to configure our equipment to function as I described, I would be very grateful.
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  1. Interesting problem.

    I don't have a great answer yet, but I would expect that your multiple routers on a single network with DHCP enabled would "compete" for assignments if not otherwise fully occupied.

    My first though is to put the routers on separate subnets, i.e. 169.254.n.1, n+1.,etc. I would then connect the PC that watches over them using connections to each subnet in multiple VMWare clients or multiple NICs, but I am sure there is a better answer than that.

    I'm really looking forward to some the of the smarter guys, like Emerald, to answer this one. :)
  2. Realbeast,

    Thanks for your thoughts; I would agree that the problem is the routers competing to assign, with such short runs, the propagation delay of the signal is so small that I imagine if the "correct" router is busy or incurs any kind of processing delay, the signal from another router likely wins the race leading to my problem.

    The controlling PC does have 2 NIC's but the second needs to be connectede to our main corporate network to save test results to a network location leaving just one NIC for this test network. The problem with using any virtual machines is that the software that controlls all the tests is one program that reaches out to all the different devices on the routers automatically, not just an address that is typed in or something to that effect. The whole setup is designed to keep things as simple as possible for the guys out on the floor, so I doubt my boss would be receptive with me changing much on the PC, but otherwise that would be a good idea.

    Idealy, my routers would allow me to assign IP based on interface port, but alass they do not, so I too am looking foward to some creative ideas.
  3. Those routers don't have many features, but I would look at IP Filtering on each of the 6 routers to isolate the "subnets," which your router supports by addresses or port ranges.
  4. The routers are definately very basic. I looked at the filtering options on them (both by IP and port) to see if I could filter out the incoming traffic from the other 5 routers (which if filtered correctly could remain on the same subnet), but it appears from the documentation in the router's help files that the filtering options on this router only block specific internal LAN IP's from accessing the Internet(WAN interface) and don't support blocking LAN traffic from the router which is really what I would need.

    I do have spare routers and switches of the same models if a proposed solution will require them, but I'm not sure that will help. I was also told by my boss this morning that If a good solution can't be found with the existing equipment, we can look into purchasing some more that is better suited to the job, although I'm sure I won't be able to spend too much on it.
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