We can into a strange problem. As above, it's a simple S2S VPN tunnel.
On our side, Node1 is primary, Node2 is secondary but connection is always established through cluster IP. There was a failover, so Node2 is primary and Node 1 is secondary.
Immediately after failover, connection to 20.20.1.1 stopped. It continued to work from Node 1(secondary) but not from Node 2 (primary).
I could see the packets from node 2 reaching our firewall and logs "Received IKEv2 request to establish an IPSec tunnel.. local traffic selector...... etc etc" but no response from remote end.
Once we logged in to remote machine and pinged Node 2, everything started working fine.
What could it be? Could we do something from VPN peers?
