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Thanigai1980
New Contributor

Overlapping Remote Subnets in the IPSEC VPN of two different Customers

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=...[/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

  • 3 REPLIES 3
    rwpatterson
    Valued Contributor III

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

    Bob - self proclaimed posting junkie!
    See my Fortigate related scripts at: http://fortigate.camerabob.com

    Thanigai1980

    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
    Honored Contributor

    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.

     

    -- 

    "It is a mistake to think you can solve any major problems just with potatoes." - Douglas Adams