Well , Typically in this case the scenario will be like the following
connect site to site vpn over isdn link , best practices are to use dedicated hardware for this purpose , based on your requirements and budget you can determine the suitable model of your router , I would recommend the use of Cisco routers , in the other hand I would recommend the use of dedicated point to point connection like leased line or at least national dsl links , After termination of your vpn connections you have the choice to connect an Ethernet switch and then to your servers and end devices .
Regards
Nader Samir
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