When deploying FortiGate devices for geographical redundancy, two primary architectural challenges must be addressed. First, a mechanism is required to redirect traffic from the primary site to the secondary site during a failure. This is typically achieved using dynamic routing protocols like BGP to withdraw routes from the failed site or using DNS-based solutions like Global Server Load Balancing (GSLB). Second, if stateful failover is required, the impact of network latency on session synchronization must be evaluated. The FortiGate Session Life Support Protocol (FGSP) can synchronize sessions over a Layer 3 network, but high latency between geographically distant sites can degrade performance and impact its feasibility.

[Assumed Option B: Requirement for a direct Layer 2 heartbeat link]

A direct Layer 2 heartbeat link is a requirement for a standard FortiGate Cluster Protocol (FGCP) high-availability cluster, which is designed for local, not geographical, redundancy.

[Assumed Option D: Use of identical hardware models]

While using identical hardware is a best practice for consistency and performance parity, it is not a fundamental architectural constraint for a geo-redundant design, which primarily concerns traffic flow and state management.

1. Fortinet FortiOS 7.6.0 Administration Guide, High Availability > HA for different geographical locations. This section explicitly states: "To fail over traffic from one FortiGate to another you could use routers that support dynamic routing (for example, using BGP) or you could use a server load balancer or a DNS server." This supports the need for a traffic redirection mechanism.

2. Fortinet FortiOS 7.6.0 Administration Guide, High Availability > FGSP. The guide notes: "FGSP can be used to synchronize sessions between two standalone FortiGates. This configuration is useful for disaster recovery, where the two FortiGates are in different geographical locations." It further advises, "The effects of latency on session synchronization performance should be considered." This confirms that latency's impact on session synchronization is a critical consideration.