Two complete networks  sharing the LAN

Posted on 2012-08-19
Last Modified: 2012-08-29
I have two different companies trying to share the same cable plant.
Each company has their own Internet connection and router.
Each company has their own workstations and printers.

So far I have been able to keep them truly apart simply by using my physical cabling routes.
Until now – please reference my attached picture. I now need to add a workstation on a location where I cannot split the cabling.

In my campus we have a Fiber link between the Gigabit Switches because we are in separate buildings.

If I try to set this up – I feel that one of the two companies simply needs to NOT utilize DHCP at all, as we do not want to share anything and of course we cannot have dual DHCP providers..

I know this might look like a basic question to some of you – so let me know if you think there are serious drawbacks to this or not. I am very open to other ideas as well
Question by:jwrotondo
    LVL 9

    Expert Comment

    From a security perspective it is a nightmare, they will be able to gain access to each other's resources and it will be difficult to prevent such without VLANs and most likely a better router to handle the routing from Network A to Network B.

    Are these two companies totally separate?  Or is it the one owner(s) but two different companies?  If it is one set of owners, perhaps they would not mind the joining of the networks.  You could pitch it as a reduction in costs, since they could drop one of the internet connections.

    Also, is it a physical barrier that is preventing you from running the new cable to this workstation?  Perhaps a new IDF closer to this new workstation can help you.  It looks like from the diagram, one side has significantly more IDFs than the fist company.
    LVL 31

    Accepted Solution

    I think at this point you need to start implementing VLANs.

    By putting all of the "A" computers and the "A" router/modem on one VLAN, and all of the "B" computers and the "B" linksys router/dsl modem on a separate VLAN, you can isolate the two networks despite the fact that they are sharing the same top level gigabit switches in your diagram.

    I've only ever done basic VLANs that are implemented on a port-by-port basis. You configure on the topmost switch which ports are VLAN1, and which ones are VLAN2. Everything stays isolated on those two networks.

    But since you have several nested layers of switches, you need probably need to utilize "VLAN tagging" to accomplish what you're looking for.
    LVL 6

    Expert Comment

    I agree, security is the primary concern. But you will also find issues with broadcast as well. In this configuration, you have a single broadcast domain with two ip subnets. Its not a proper configuration.


    Author Closing Comment

    Although I do not setup VLANS everyday this was clearly the way to go on this project. Thanks for everyones help on this issue

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