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There are two (currently separated) segments of the same intranet (same subnet). There's a router in different subnet both mentioned segments can connect to. In pseudographics:

Intranet      <-> Gateway A      Router       Gateway B  <->  Intranet
192.168.10.0/24   192.168.10.1                192.168.10.2    192.168.10.0/24
                  10.0.0.2   <-> 10.0.0.1 <-> 10.0.0.3

Is it possible to unite both intranet segments without VPN tunnel, using static routes?

If that's important, both gateways are CentOS-running computers.

Thanks.

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Is it safe to assume Gateway A and Gateway B are too far apart to run a cable between them? –  Justin Pearce May 21 '12 at 16:00
    
2 Justin Pearce: they are several kilometers away from each other. –  Konstantin Boyandin May 22 '12 at 8:36
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2 Answers

Because the same subnet is used on both Ethernet domains, no, this cannot be done with layer 3 routing. You need a layer 2 solution in order to achieve this. If Gateway A and Gateway B are on the same Ethernet domain and don't actually use the router to communicate with each other, you can achieve this by bridging the two Intranets onto the Ethernet domain shared by the gateways (but you're essentially forming a single Ethernet domain, which isn't preferable). If the gateways must use router then you need some kind of layer 2 tunnel between the gateways. The best solution is to renumber one of the Intranets so you can use layer 3 routing.

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Thanks, but the subnets on both sides should be the same. It should be the same intranet, where all the hosts are seen to one another. –  Konstantin Boyandin May 22 '12 at 8:38
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It is possible to do, but with some problems: it's more difficult to maintain, inoptimal routing problem - traffic destined to 192.168.10.0/24 will go to either A or B gateway, passing tunnel if it need to go to another location.

Consider to do IP renumbering. But if this is not possible:

First option is tunneled bridging:

You need a tunnel between gateways A and B. If your routers allows you to create some L2 over IP tunnel - you can bridge this tunnel to LAN interface.

Another option is:

  1. create L3 tunnel - e.g. GRE.
  2. create proper routings to IPs on your LAN, for example if you have 192.168.0.10 on location A, you create route to this ip to LAN interface on gateway A, and to tunnel interface on gateway B.
  3. set proxyarp feature on both LAN interfaces and tunnel interface
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Thanks. Looks like tunnel bridging is the optimal solution. –  Konstantin Boyandin May 22 '12 at 9:01
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