Use the Dead Peer Detection check box to enable or disable traffic-based dead peer detection. When you enable dead peer detection, the Firebox connects to a peer only if no traffic is received from the peer for a specified length of time and a packet is waiting to be sent to the peer. This method is more scalable than IKE keep-alive messages.

Nov 05, 2017 Configuring the Peer VPN gateway | Cloud VPN | Google Cloud Jun 26, 2020 [Solved] The peer is not responding to phase 1 ISAKMP requests Apr 20, 2020

The error, "Secure VPN connection terminated locally by the Client. Reason 412. The remote peer is no longer responding". Means that the software VPN Client detected that the VPN server is not responding anymore and deleted the connection. Now, this is caused by several different issues, for example:

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VPN Availability Configuration Guide, Cisco IOS XE

If you're using the peer cache feature with roaming peer cache sources, after updating the site to version 1806, also update all peer cache sources to the latest client version. The management point doesn't include these peer cache sources in the list of content locations until they are updated to at least version 1806.