Problem Radius over VPN

[Luca Bertoncello]

Hi list!

I already reported in March my problems using Freeradius over VPN.
I spent many time searching the problem, and maybe I found something, but I have no idea how to correct the problem...

So, short explanation:
Main office with Freeradius, connected via OpenVPN to our central VPN-Server.
Second office with the AccessPoints, connected via OpenVPN to our central VPN.
"Normal" pakets from second office to main office (and viceversa) go through both VPNs.

Now, I sniffed the pakets on all servers (VPN server on second office, central VPN server, VPN server on main office and Freeradius), and I discovered that some pakets are blocked.

Wireshark on VPN server of the second office:

12	2022-03-25 14:39:48,154608	10.0.21.10	10.6.21.10	RADIUS	979	Access-Challenge id=86
13	2022-03-25 14:39:48,476555	10.6.21.10	10.0.21.10	IPv4	1500	Fragmented IP protocol (proto=UDP 17, off=0, ID=41c1)
14	2022-03-25 14:39:48,507473	10.0.21.10	10.6.21.10	RADIUS	92	Access-Challenge id=87
15	2022-03-25 14:39:48,533183	10.6.21.10	10.0.21.10	IPv4	1500	Fragmented IP protocol (proto=UDP 17, off=0, ID=41c2)
16	2022-03-25 14:39:51,533406	10.6.21.10	10.0.21.10	IPv4	1500	Fragmented IP protocol (proto=UDP 17, off=0, ID=42d9)

Wireshark on central VPN:

12	2022-03-25 14:39:48,129787	10.0.21.10	10.6.21.10	RADIUS	979	Access-Challenge id=86
13	2022-03-25 14:39:48,488993	10.6.21.10	10.0.21.10	IPv4	1500	Fragmented IP protocol (proto=UDP 17, off=0, ID=41c1)
14	2022-03-25 14:39:48,501213	10.0.21.10	10.6.21.10	RADIUS	92	Access-Challenge id=87

No other pakets after paket 14 (ID 87) reach the central VPN serve, so the problem "must be" either on the central VPN server or on the VPN server oft he second office...
Now the very question: do someone have an idea why just the first fragmented paket run over all VPNs and the other one do not?

Thanks a lot
Luca