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Thanigai1980
Thanigai1980
New Member
October 20, 2021
Question

Overlapping Remote Subnets in the IPSEC VPN of two different Customers

  • October 20, 2021
  • 1 reply
  • 4711 views

Hi All,

 

I am facing issue with the VPN IPSEC tunnel as my remote subnet of two different customers has route conflict.

 

As you all know, the customers never accept changes at their end which is an unwritten rule, so I have tried the below options to make it work but no luck.

 

[ul]
  • DNAT for Overlapping subnets.
  • VRF option
  • VRF with VDOM link option (https://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD47283&sliceId=1&docTypeID=DT_KCARTICLE_1_1&dialogID=259122736&stateId=1%200%20259124276%27)[/ul]

    But, nothing worked, still I am facing the same issue. Can anyone recommend me the right option to fix this.

     

    Note - The link I have shared in the third bullet is the same topology of my scenario.

     

    My end - Fortigate 1500D--------------------> Customer End - Cisco Meraki

     

    Regards,

    Thanigai

      • 1 reply

      rwpatterson
      rwpatterson
      New Member
      October 20, 2021

      Force them to NAT to an unused range. I know they hate change, but if they need you, they'll do it.

      Thanigai1980
      Thanigai1980Author
      New Member
      October 22, 2021

      Hi Bob,

       

      Thanks for your inputs.

       

      I made them to NAT their Subnet from 192.168.1.0/24 to 172.20.1.0/24 and the traffic is flowing fine now.

       

      But, I got a new challenge, as they are syncing Active Directory between the sites, and it doesn't support NAT.

       

      Microsoft also not supporting this model. Now I am back to zero and have to find some solution again.

       

      Regards,

      Thanigai.R

      sw2090
      SuperUser
      sw2090
      SuperUser
      October 26, 2021

      There is some kb article on this: https://kb.fortinet.com/kb/documentLink.do?externalID=FD35117

      They use SSLVPN here but that will also work with IPSEC.

      The trick here is to use vip to remap ips to a virtual subnet to work around the overlapping.

       

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