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

● Theneighbor-groupparameter is used to apply pre-defined settings (such as AS number) to dynamically discovered BGP neighbors.

● Theadvpnneighbor-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 allowsdynamic BGP peer discoveryby defining a range of potential neighbor IPs (172.16.1.1 - 172.16.1.255).

● Sinceeach spoke has a unique /32 IPwithin this subnet, this ensures that any spoke within the172.16.1.0/24range can automatically establish a BGP session with the hub.

When configuring aloopback interface as the BGP sourceforconnecting to an ISP, two important settings must be applied:

1.Enable EBGP Multihop (ebgp-enforce-multihop)

BGP normally expects directly connected neighbors, but since the ISP and FortiGate A are usingloopback interfaces,packets will not be sent directly between their physical interfaces.

Theebgp-enforce-multihopcommandallows BGP to form an eBGP peering over multiple hops.

2.Set the Update Source (update-source)

Since FortiGate is using aloopback interface as the source, theupdate-sourcecommand ensures thatBGP updates originate from the loopback interfacerather than a physical interface.

This is essential becauseBGP peers must match the source IP with the configured neighbor address.

ThisIPsec Phase 1 configurationdefines adynamicVPN tunnel that can accept connections from multiple peers. The settings chosen here suggest a configuration optimized fornetworks with intermittent traffic patternswhile ensuring resources are used efficiently.

Key configurations and their impact:

●set type dynamic→ This allows multiple peers to establish connections dynamically without needing predefined IP addresses.

●set ike-version 2→ UsesIKEv2, which is more efficient and supports features like EAP authentication and reduced rekeying overhead.

●set dpd on-idle→ Dead Peer Detection (DPD) is triggeredonly when the tunnel is idle, reducing unnecessary keep-alive packets and improving resource utilization.

●set add-route enable→ FortiGate automatically adds the route to the routing table when the tunnel is established, ensuring connectivity when needed.

●set proposal aes128-sha256 aes256-sha256→ Uses strong encryption and hashing algorithms, ensuring a secure connection.

●set keylife 28800→ Sets alonger key lifetime(8 hours), reducing the frequency of rekeying, which is beneficial for stable connections.

BecauseDPD is set to on-idle, the tunnel will not constantly send keep-alive messages but will still ensure connectivity when traffic is detected. This makes the configuration ideal fornetworks with regular but non-continuous traffic, balancing security and resource efficiency.