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@Ambuj M You should have these commands available, indeed from 17.15.X:
Step 1. Create TACACSS server and AAA groups, associate the client (router) trustpoint.
tacacs server svs_tacacs address ipv4 10.225.253.209 single-connection tls port 6049 tls i...
There is no real "HA" setup, you simply install two DUO Authentication proxy in your environment. This way you have two servers providing the DUO service. You then point your relevant devices to these duo servers and that is it.
That depends on the Authentication Source going to the Proxy. Normally devices (such as ASA or other Firewalls) allow you to enter multiple radius servers in their configuration.
What device are you looking to set up?
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