To be able to advertise the same subnet you can not have a direct connection to it. So if the production site was using 192.168.255.0/30 to connect to the layer3 switch at the site, and the DR site used 192.168.255.4/30 to connect to the layer3 switch at that site, then both MXs can advertise connectivity to 192.168.1.0/24. I'm not completely sure on this one - but I believe whichever MX/network was added to the org first becomes the primary for the route. The DR route is only used if the primary MX goes offline.
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