The negotiation of a VPN connection is based on the creation of keys according to the Diffie-Hellman method. Depending on the key length, however, calculating a DH key can take some time. If a VPN gateway with 100 VPN connections temporarily loses its Internet connection, recalculating these 100 DH keys may take a while before all of the VPN clients can establish a connection to the VPN gateway again.
With the precalculation of DH keys, the device can accelerate the renegotiation of VPN connections:
The precalculation of keys has a low priority. When the CPU of the device is not busy with other, higher priority tasks, it builds up a stock of public DH keys.