I found this a neat tidbit of knowledge that I thought I should share.
Created a Fortinet SDWAN setup with many sites – works awesome. Fortinet’s SDWAN is simple, but very effective with some really cool features to help with SLAs.
During this setup, I have Radius, and LDAP servers located across one of the members of the SDWAN (An IPSEC tunnel). As before with tunnels, you would modify the source address in CLI for the destination server. This would make sure requests originate from the interface you are expecting, and will go through the tunnel.
After switching the tunnel to be a member of a SDWAN interface the LDAP/RADIUS requests stopped working. I found another command was needed. This command is needed not only for Radius, and LDAP but also for FAZ, Fortiguard, and dns.
In CLI of both Radius and LDAP servers you will see these options:
The trick here is to set the “interface-select-method” to sdwan. This resolved my issue and allowed things to start working. The reason it does this is that Self-originating traffic, such as Syslog, FortiAnalyzer logging, FortiGuard services, remote authentication, and others, relies on routing table lookups to determine the egress interface that is used to initiate the connection. Policy routes generated by SD-WAN rules do not apply to this traffic.