This article applies to the following firmware versions and product models: VPN, FAQ.
What does the ‘Enable OCSP Checking’ checkbox do? (Under VPN > Advanced tab).
OCSP, short for Online Certificate Status Protocol, is a real-time method of checking the validity of a digital certificate during VPN negotiation. This option is used when certificates have been used as the authentication mechanism for a VPN Policy; if enabled, the SonicWall will contact a remote validating entity (configured in the ‘OCSP Responder URL’ sub-option) to see if the certificate sent by the remote peer is valid.
Source: SonicWall IKE / IPSec VPN Implementation FAQ