The base BFD specification does not specify the encapsulation type to be used for sending BFD control packets. Instead, use the appropriate encapsulation type for the medium and network. The encapsulation for BFD over IPv4 and IPv6 networks is specified in RFC 5881, Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop), and RFC 5883, Bidirectional Forwarding Detection (BFD) for Multihop Paths, BFD for IPv4 and IPv6. This specification requires that BFD control packets be sent over UDP with a destination port number of 3784 (single hop) or 4784 (multi-hop paths) and the source port number must be within the range 49152 to 65535.
Also, the TTL of all transmitted BFD packets must have an IP TTL of 255. All BFD packets received must have an IP TTL of 255 if authentication is not enabled. If authentication is enabled, the IP TTL should be 255, but can still be processed if it is not (assuming the packet passes the enabled authentication mechanism).
If multiple BFD sessions exist between two nodes, the BFD discriminator is used to de-multiplex the BFD control packet to the appropriate BFD session.