Comprehensive and Detailed 150 to 200 words of Explanation From Exact Extract of Enterprise Firewall 7.6 Administrator documents:
According to the FortiOS 7.6 Administration Guide and the FortiGate Infrastructure study materials, inter- VDOM communication can be achieved using either software-based VDOM links or hardware-accelerated NPU VDOM links (vlinks).
While standard VDOM links (Option B) allow traffic to pass between VDOMs, they are processed by the system CPU, which can become a bottleneck in high-throughput environments. To ensure accelerated internet access as specified in the requirements, NPU vlinks (Option C) must be used. NPU vlinks are virtual interfaces created in pairs that allow traffic to be offloaded to the FortiGate's Network Processor (NP6, NP7, etc.), significantly reducing latency and CPU overhead.
In the provided exhibit, the root VDOM has direct internet access, while VDOM1 and VDOMn do not. By configuring NPU vlinks between the non-root VDOMs and the root VDOM, you create a hardware- accelerated path. Traffic from the internal VDOMs is sent through the vlink to the root VDOM, which then forwards it to the Internet. This " hub-and-spoke " VDOM architecture, powered by NPU acceleration, ensures that all VDOMs share the internet connection without sacrificing performance.

For the hub to automatically create iBGP neighbors for the spokes in this ADVPN, the neighbor- range must match the spokes' overlay IP addresses (172.16.1.2 and 172.16.1.3), so the prefix is set to 172.16.1.0/24. The neighbor-range must also be tied to the preconfigured neighbor-group advpn so that all dynamically created spoke neighbors inherit the BGP settings (including remote- as 65100).

In the given ADVPN (Auto-Discovery VPN) topology, BGP is being used to dynamically establish routes between spokes. The neighbor-range configuration is crucial for simplifying BGP peer setup by automatically assigning neighbors based on their IP range.
set neighbor-group advpn
The neighbor-group parameter is used to apply pre-defined settings (such as AS number) to dynamically discovered BGP neighbors.
The advpn neighbor-group is already defined in the configuration, and assigning it to the neighbor- range ensures consistent BGP settings for all spoke neighbors.
set prefix 172.16.1.0 255.255.255.0
This command allows dynamic BGP peer discovery by defining a range of potential neighbor IPs (172.16.1.1 - 172.16.1.255).
Since each spoke has a unique /32 IP within this subnet, this ensures that any spoke within the
172.16.1.0/24 range can automatically establish a BGP session with the hub.