Which protocol is used to send update message in eigrp?

EIGRP uses RTP for Reliable communication..

EIGRP its self is a protocol. EIGRP uses5 different types of Packets.

(Hello, Update, Query Reply, Acknowledge)

Hello packets are used to from neighbour relationships.

Update packets: after hello packets and neighbor relation ships are formed. update packets are used to exchange routing tables

Query: if the sucessor is down and there is no feasible sucessor. then query packets are used to search for the next path from its neighbors

reply: when a router recieve a query from its neighbor it will reply it back

acknowledge: they are used for relaiblility of data. to make sure that packets are reaching their destinations. 

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Hope this will help you to understand about EIGRP