Reference

https://community.fortinet.com/fortigate-3/troubleshooting-tip-dhcp-status-removed-due-to-conflict-137169

Situation

[1]
# FGT發DHCP卻一直顯示Removed due to conflict
The FortiGate is acting as the DHCP server, but it keeps showing the message "Removed due to conflict".

[2]
# 使用debug flow監聽DHCP的DORA
FGT Login > CMD
> diagnose debug reset
> diagnose debug application dhcps -1
> diagnose debug enable

[3]
# 觀察配發DHCP時測試該IP有沒有人使用，每一個IP都有人回應
[debug]Sending ICMP echo-request to 192.168.180.4
[debug]Received ICMP echo-reply from 192.168.180.4
[warn] Abandoning IP address 192.168.180.4: pinged before offer

Solution

[1]
# 檢查VIP裡面的ARP Reply有沒有開啟
# 勾選ARP reply代表FGT會回應VIP Pool池的IP，剛好跟DHCP Pool衝突就會讓防火牆回應ARP，導致無法配發終端IP
FGT WebUI > Policy & Objects > Virutal IPs > ARP reply disable


[2]
# 7.6版本之後可以開啟IP衝突的事件紀錄
FGT Console 
> config system global
> set ip-conflict-detection enable
> end

Check

diagnose debug reset
diagnose debug application dhcps -1
diagnose debug enable

diagnose debug disable