IP Address Management (IPAM) in FortiSASE is responsible for automatically allocating subnets to various services, including VPN tunnels and Edge devices. When an administrator modifies the default IPAM configuration, they must adhere to specific architectural scaling requirements.

Subnet Requirements per PoP: FortiSASE architecture requires a minimum amount of address space to be available for each provisioned Security Point of Presence (PoP) to handle internal routing and endpoint assignments. For the IPAM engine to function correctly and distribute unique subnets across the global infrastructure, the pool must provide at least one /20 subnet per security PoP . If the available space is smaller than this per-PoP requirement, the allocation logic may fail or produce unpredictable routing behavior.

Impact of Excessive Exclusions: In the exhibit (image_578940.png), the customer has defined a large summary pool of 172.16.0.0/12. However, they have configured eight separate /15 excluded subnets: 172.16.0.0/15, 172.18.0.0/15, 172.20.0.0/15, 172.22.0.0/15, 172.24.0.0/15, 172.26.0.0/15, 172.28.0.0/15, and 172.30.0.0/15.

Calculating the Exhaustion: A /12 network contains exactly eight /15 blocks. By excluding all eight /15 ranges listed in the exhibit, the customer has effectively excluded 100% of the available addresses from the primary 172.16.0.0/12 pool.

Connectivity Problems: When the IPAM pool is exhausted or overly restricted, FortiSASE cannot assign valid, non-overlapping subnets to the PoPs. This leads to connectivity problems for remote users and can cause the system to " fall back " to ranges it believes are available, even if they were intended to be excluded, or simply fail to establish tunnels entirely.

To resolve this, the administrator must ensure that the excluded subnets do not consume the entire pool and that the remaining unexcluded space is large enough to provide a /20 block for every active PoP in their subscription.