ADSL lines in Germany are brought down once every 24 hours on purpose, at least with German Telekom. As ADSL is targeted and marketed as a broadband medium for private persons this is meant to defeat the use of these lines for servers - the customer will be assigned a new public IP every 24 hours.
So your logs only show that the VPN was established between an ADSL line and a HDSL line (without forced disconnections). If you set up all parameters correctly the tunnel will be reestablished within seconds.
To make your VPNs fully and automatically redundant, you may already have set the 'monitor-phase1' parameter in the backup VPN setup. Given a name of the main VPN FortiOS will monitor it for failures and yank the backup VPN up in that case.
I didn't know in Germany they brought down VPN on purpose. That's ok no problem.
I know I have one of the interface that is an ADSL and this kind of line is not well suitable for business ( but this is just a backup of the HDSL line we have in Germany and for this ADSL we have a STATIC public IP assigned, so no problem about IP change ).
You are right, the VPN is re-established within seconds.
And to make our VPNs fully and automatically redundant we are using different "Distance" value in the Static Routes configuration (and it is working well).
This VPN between 220.127.116.11 ( one of the 2 HDSL in Italy) and 10.1.2.2 ( Germany ADSL that have a public STATIC ip 18.104.22.168), is just the 4th IPSEC VPNs we have and the least important.
Infact, we are going to use this only in case the others 3 will have a problem.
And my little problem rise here.
Why, in the others 3 IPSEC VPN, I don't see so many "IPsec DPD failure" messages.
I was thinking, maybe it is the new HDSL we just installed here in Italy that can have some problems ...
but at the same time this new HDSL (22.214.171.124) having 2 VPNs :
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