RADIUS Communications Architecture
RADIUS uses the UDP protocol for its communication needs. Although some early implementations of RADIUS used port 1645, the official UDP port to use for RADIUS is 1812.
The preceding chapter looked at some of the advantages of using TCP versus UDP for transmitting TACACS+ information. However, arguments based on the specific needs of the RADIUS protocol advocate the use of UDP for RADIUS communications. The following is a quote from RFC 2865, explaining the appropriateness of UDP as the transport mechanism for RADIUS:
1. If the request to a primary Authentication server fails, a secondary server must be queried. To meet this requirement, a copy of the request must be kept above the transport layer to allow ...