This issue only occurs on FortiOS 7.2.x. While FortiOS has a radius profile with two servers (primary, secondary), it might not fail over to the secondary radius server, when primary is not reachable.
For instance, assume that there are two RADIUS servers from two different destination subnets, which are routed accordingly from tunnel interfaces. There may be a ‘blackhole’ route for the subnet of the primary server. When all tunnels are down, there may be no route to the primary RADIUS server. This exact event does not trigger failover to the secondary RADIUS server.
Either of the following error snippets may show in logs:
2026-05-21 11:15:32 [1419] fnbamd_rad_dns_cb-10.10.20.76->10.10.20.76
2026-05-21 11:15:32 [1388] __fnbamd_rad_send-Error sending radius request: fd=10, Network is unreachable
Or:
2026-05-22 20:41:03 [1419] fnbamd_rad_dns_cb-172.20.30.23->172.20.30.23
2026-05-22 20:41:03 [1388] __fnbamd_rad_send-Error sending radius request: fd=11, Invalid argument
This issue only exists in the 7.2.x branch of FortiOS. FortiOS running on 7.4 and 7.6 fails over to the secondary server successfully. Since 7.2 is out of engineering support, it will not be fixed in this major version.
As a solution, consider the following workaround: use two separate RADIUS servers added to the same group as a source. FortiGate sends requests to both RADIUS servers, and the first response will be considered:
2026-05-22 20:55:17 [1939] handle_req-Rcvd auth req 405070729 for raduser01 in ggg opt=00000000 prot=0
2026-05-22 20:55:17 [489] __compose_group_list_from_req-Group 'ggg', type 1
2026-05-22 20:55:17 [616] fnbamd_pop3_start-raduser01
2026-05-22 20:55:17 [573] __fnbamd_cfg_get_radius_list_by_group-Loading RADIUS server 'radius' for usergroup 'ggg' (3)
2026-05-22 20:55:17 [573] __fnbamd_cfg_get_radius_list_by_group-Loading RADIUS server 'radius-failing' for usergroup 'ggg' (3)
2026-05-22 20:55:17 [592] __fnbamd_cfg_get_radius_list_by_group-Loading RADIUS server 'radius' for usergroup 'ggg' (3)
Related article:
Using multiple RADIUS servers | 
