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We currently have 2 networks utilising the same 2 switches (so a computer attached to either switch can connect to either network), and we need to add in a 3rd switch but we’re not sure exactly how to connect this. The structure we have at the moment works 100%, but we tried to add in a 3rd switch and 1 of the routers fell over temporarily, so we’ve removed the 3rd switch and are now asking for help.

The set-up at the moment is:

  • Router A (192.168.10.1) connects to Switch A
  • Router B (192.168.55.1) connects to Switch B
  • Switch A connects to Switch B

Quite simply, any server plugged into switch A or B can connect to either of the routers and so it can join either of the networks (we control this in the network connection settings on each box). This all works 100% and everything functions exactly as we require. We have run out of available network ports on the 2 switches, so we need to add a 3rd switch (Switch C) into this structure, but we’re not sure of the best way to do this. We tried connecting switch C to both routers directly, and 1 of the routers went offline immediately. As soon as we pulled switch C out, the router was fine again (so clearly this is not a good way to do it). Routers are both Safe@Office 1000N and switches are unmanaged Netgear 16 and 24 port switches.

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3 Answers 3

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The simplest solution is to connect switch C to either of switch A or B via a single link. Once you have done that you can plug stuff into Switch C and it will see both networks.

Connecting it to bother routers as you did would cause problems

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  • Phil, very clear. Thank you. The 1 concern I have about doing it that way (i.e. single connection to switch A or B) is that if the switch A or B goes down, then switch C can't communicate. Is it possible to connect switch C to both switch A and B at the same time, so that there is a level of protection ?
    – Roydon
    Jul 23, 2013 at 13:26
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    @Roydon - Not with unmanaged switches. You'll create a switch loop, which will most likely bring the network down.
    – joeqwerty
    Jul 23, 2013 at 13:28
  • OK, so we have 2 options then: 1 - Connect switch C to either of switch A or B, OR 2 - Connect switch C between switches A and B (so C sits between A and B) Which is the better option ?
    – Roydon
    Jul 23, 2013 at 13:37
  • There is not much in it - both will still give you a single point of failure I'm afraid
    – Phil
    Jul 23, 2013 at 14:06
  • Yeah, can't get away from that. Thanks everyone, much appreciated !
    – Roydon
    Jul 23, 2013 at 14:15
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If all of your switches are unmanaged then your mistake was connecting the third one to both switches. Just connect it to one.

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Ethernet by definition does not allow for loops in the topology.

Connect your additional switch accordingly, for example to just one Router.

If you want to balance/fail-safe your Network, have a look at STP. It will allow you to create physical loops, but the resulting L2 topology will be compliant.

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