IP Conflict between Fortigate and a Server (Strange Issue)

Hi,

Ideally, When an IP address is assigned to a network interface, the operating system first checks for IP conflicts by sending an ARP probe, which is essentially an ARP request. If no conflicting responses are received, it assigns the IP address to the NIC and then announces it to the network using Gratuitous ARP (GARP).

If you find the FortiGate LAN interface's MAC address associated with the IP address in the server's ARP table, it means the FortiGate has responded to the ARP probe sent by the server during IP assignment. This suggests that the firewall owns the server's IP address and is responding to ARP requests.

I would recommend checking if the server's IP address is used in the firewall's configuration, particularly in settings like IP pools, proxy ARP, VIP (Virtual IP), etc. You can use the following command to verify this:

# sh | grep <server IP address>

If they are used in the config, then you could either remove the settings from the firewall or disable arp-reply under the settings (only if used on IP-Pool or VIP).
You could refer to the article in the below link to perform the same,
https://community.fortinet.com/t5/FortiGate/Technical-Tip-ARP-reply-setting-in-Virtual-IP-IP-Pool/ta-p/192527

Cheers,