| As well as know configuring server redundancy is the best practice in case of remote authentication(especially for the administrator users)
Check the below-mentioned article, there are Two radius servers configured.
one is acting as Primary while the other one is Secondary.
There is also remote administrator configured, check below.
So whenever Admin users tries to login he will authenticated through primary server and if the primary server is down then authentication will be handled by the secondary server.
Note.
The user must be a part of both servers.
Check the below-mentioned sniffer.
Sniffer 1 >>> Sucessfull authentication(Through primary server).
filters=[(host 172.31.133.140 or host 10.133.5.120) and port 1812 ]
14.321084 port2 out 172.31.133.115.15201 -> 172.31.133.140.1812: udp 111
14.337001 port2 in 172.31.133.140.1812 -> 172.31.133.115.15201: udp 20
Sniffer 2 >>> Sucessfull authentication(Through primary server) when primary is down.
16.139606 port5 out 10.133.5.115.1429 -> 10.133.5.120.1812: udp 111
16.149698 port5 in 10.133.5.120.1812 -> 10.133.5.115.1429: udp 20
From the output, traffic is going through secondary one. | 
