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Now: SW02 currently running OSPF area 0 with redistribute connected subnets which includes VLAN10 with another router/switches which isn't in the picture and not in the scope of this question; VLAN10 contain 10.0.0.1 as gateway for Desktop Group 1 and 2

Objective: We wanted to move Desktop group 2 to site B, since we want IP routing handle individually on site B, which is SW04, we are planning to put 10.0.0.2 as VLAN10 interface IP and that make it become the gateway for Desktop group 2 user in site B, while SW02 and SW04 is running OSPF area 0.

Question: we tested to distribute 10.0.0.0/24 in area 0 between SW02 and SW04, but it seems only being able to handle one subnet with one site only, is there a limitation for route injection not to have same subnet between two sites?

FYI, please don't ask me why it needs to be like this or could it completely change the design, I don't make the rule, that's the business requirement, thanks.

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  • Routers route packets between networks, not from a network back to the same network. A host will never send a packet to the router if it is in the same network, so you will never route traffic to the other (same) network. You route between networks, but bridge on the same network. It is not a good idea to bridge between different sites because you introduce a big delay in a layer-2 domain that really expects minimal latency, and any layer-2 problems at one site affect both sites. – Ron Maupin Jul 27 '20 at 0:02
  • @RonMaupin so that means it wouldn't work in this setup, correct? – Thomas G. Lau Jul 27 '20 at 0:14
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    Not with layer-3 routing. You would need to do something ugly like using NAT for overlapping networks, but that will break OSPF. – Ron Maupin Jul 27 '20 at 0:14
  • @RonMaupin OK please put it as an answer stated this wouldn't work, so I could close this, thanks. – Thomas G. Lau Jul 27 '20 at 0:39
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What you want to do will not work the way you envision. You cannot have overlapping networks separated by layer-3 without an ugly workaround, e.g. NAT. The problem with that is NAT will break OSPF, among other things.

To use the same network and OSPF, you would need to bridge the two sites at layer-2, and that can cause other problems, such as creating latency in a layer-2 domain, and propagating any layer-2 problems at one site to the other site.

The proper way to do it is to simply use different network addressing at the second site.

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