Reference

https://community.fortinet.com/t5/FortiGate/Technical-Tip-OSPF-MTU-mismatch-after-upgrade/ta-p/197883

Situation

# 預設OSPF MTU從6.4.6.升級成7.0.1後改變了
Default OSPF MTU is been changed after [Patch 7.0.1].
6.4.6 : 1438
7.0.1 : 1420

Check OSPF interface MTU

# 確認OSPF interface MTU
get router info ospf interface

Solution

[1]
Login FortiGate WebUI > CLI

[2]
＃ 設定OSPF介面的MTU 或是 不確認MTU即可形成adjacency
config router ospf
    config ospf-interface
        edit "interface"
            1. set mtu xxxx
            2. set mtu-ignore enable
        next
    end
end