The MAC address e0:23:ff:fc:00:86 follows the format used in FortiGate High Availability (HA) clusters.
When FortiGate devices are in an HA configuration, they use virtual MAC addresses for failover and redundancy purposes.
The suspicious packet is related to a cluster that has VDOMs enabled:
FortiGate devices with Virtual Domains (VDOMs) enabled use specific MAC address ranges to differentiate HA-related traffic. This MAC address is likely part of that mechanism.
The suspicious packet is related to a cluster with a group-id value lower than 255:
FortiGate HA clusters assign virtual MAC addresses based on the group ID. The last octet (00:86) corresponds to a group ID that is below 255, confirming this option.

The OSPF database optimization is necessary to reduce unnecessary routing information and improve network performance. In the given topology, Area 0.0.0.1 is a non-backbone area connected to Area 0.0.0.0 (the backbone area) through an Area Border Router (ABR).
To optimize OSPF in this scenario, configuring Area 0.0.0.1 as a Stub Area will:
Reduce the size of the OSPF database by preventing external routes (from outside OSPF) from being injected into Area 0.0.0.1.
Allow only intra-area and inter-area routes, meaning routers in Area 0.0.0.1 will rely on a default route for external destinations.
Improve convergence time and reduce router processing load since fewer LSAs (Link-State Advertisements) are exchanged.

Zero phase selectors in IPsec Phase 2 mean that no specific traffic selectors (subnets) are defined, allowing any traffic to be encrypted through the VPN tunnel. This can cause unintended traffic forwarding issues and disrupt network operations.
To prevent this from happening during future migrations:
Using routing protocols ensures that only specific subnets are advertised over the tunnel.
Dynamic routing (such as OSPF or BGP) helps define which networks should use the tunnel, preventing unintended traffic from being encrypted.
Clearly defining phase 2 selectors avoids the problem of encrypting all traffic by explicitly stating the allowed source and destination subnets. This prevents the tunnel from affecting unrelated network traffic.

To minimize CPU and RAM usage while still enforcing security features like web filtering and application control, SSL certificate inspection mode is the best choice. SSL certificate inspection allows FortiGate to inspect only the SSL/TLS handshake, including the Server Name Indication (SNI) and certificate details, without decrypting the full encrypted payload. This enables features like web filtering and application control because FortiGate can determine the destination website or application based on SNI and certificate information. It significantly reduces system load compared to full SSL inspection, which requires full decryption and re-encryption of traffic.

Comprehensive and Detailed Explanation From Exact Extract documents and Knowledge:
The TCP Maximum Segment Size (MSS) defines the largest payload (the data portion of the segment, excluding TCP and IP headers) that a device can receive in a single TCP segment.
When you configure tcp-mss-sender and tcp-mss-receiver on a FortiGate interface or tunnel, the FortiGate intercepts the TCP 3-way handshake (SYN and SYN-ACK packets). It modifies the MSS value announced by the hosts to ensure that the resulting segments, once encapsulated (e.g., by IPsec or VXLAN), do not exceed the MTU of the path. By reducing the allowed payload size at the source, FortiGate prevents the need for fragmentation further down the line, which improves overall network efficiency and prevents packet loss in " MTU-tight " environments.