A remote branch site is reporting intermittent connectivity to the Data Center.

Comprehensive and Detailed Explanation

To diagnose specific VPN negotiation failures (Phase 1 or Phase 2 IPSec issues), the Event Logs (specifically filtered for System or VPN events) are the correct resource.

Event Logs: This section records the control plane signaling messages. If a VPN tunnel fails to establish, the Event Log will generate an entry containing the specific IKE failure reason sent by the peer or generated locally. Common errors found here include INVALID_COOKIE, NO_PROPOSAL_CHOSEN (mismatch in encryption algorithms), or PRE_SHARED_KEY_MISMATCH.

Flow Browser (A): This shows user traffic (TCP/UDP sessions). If the VPN is down, user traffic won't even enter the tunnel, so the Flow Browser will just show dropped flows or blackholes, but it won't explain why the tunnel itself is broken.

Link Quality (D): This shows latency/loss graphs for established tunnels. It cannot diagnose why a tunnel failed to form in the first place.