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guharlaga
guharlaga
New Member
June 14, 2026
Question

Weird issue with ISP modem

  • June 14, 2026
  • 3 replies
  • 49 views

Having an issue I’ve never seen before, I have a ticket open with TAC but figured I’d see if anyone has ever seen this before. As soon as I plug the modem into my Fortigate, the modem crashes constantly and will never get a lock on the ISP signal. As soon as I unplug it and connect a laptop directly to the modem, it is able to establish a lock and I get a public IP. This occurs both if I try to get a public IP using DHCP or if I try to set one of our static IPs. The ISP has tried two of their Router/Modem combo units, as well as we have tried two Arris Surfboards, one of which is known working on a different circuit. Thanks for any help!

    3 replies

    msanjaypadma
    Staff
    msanjaypadma
    Staff
    June 14, 2026

    Hi ​@guharlaga ,

    If the modem is experiencing frequent crashes, this behavior seems unusual. Does this occur when connecting a single laptop? How often do these crashes happen? Did you check with respective vendor for modem crash issue?
     

    Regarding the internet connectivity issue and failure to obtain an IP address from the ISP, could you please confirm whether the FortiGate device is configured in High Availability (HA) mode? If it is an HA device, have you configured a specific HA group ID? 

    If the device is set to the default HA configuration, there may be a MAC address conflict within the ISP network. Since ISP server uses your device's MAC address for IP assignment, such conflicts could prevent proper IP allocation. 

    Can you configure a random HA group ID on the FortiGate device and then check the current status?

    example:

    config system ha
        set group-id 120 <----- (XX is an integer value from 0-255).
    end


    Refer below article : 


    If you have found a solution, please like and mark it as solved to make it easily accessible for everyone.
     

    Thanks, Mayur Padma
    sjoshi
    Staff
    sjoshi
    Staff
    June 15, 2026

    Hi ​@guharlaga 

     

    When you connect modem to the FortiGate, are you able to access the modem?

    Is the FortiGate and modem interface are up?

    Is the FortiGate in HA?

    Is the issue only with reaching the modem gateway with interface being up?

    Thanks, Salon
    gstefou
    gstefou
    Explorer
    June 15, 2026

    HI ​@guharlaga, 

     

    I’ve seen many cases where the ISP locks the public IP address and internet access by the MAC address of the device you initially connect to their modem. Plug your firewall into their modem and give them a call to find out if they can release the MAC address bound to their equipment and reserve the WAN interface of your firewall.  

    In addition, make sure of the configuration details they require based on their setup. Such as, negotiation speed, auto or full duplex (I’ve seen ISPs asking to manually set this to 1G Full Duplex) 

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