In general, a hello message serves the purpose of announcing the (continuous) presence of a device on a network. However, that is everything that can be said in general. Every protocol uses its own distinct and different hello message contents and there are lots of protocols using the hello mechanism. Are you interested in a particular protocol?
The word "hybrid" in relation to EIGRP is a great misunderstanding and a result of dubious marketing strategy. The EIGRP was originally marketed under the term "hybrid balanced routing protocol" but from a fundamental point of view, calling the EIGRP a hybrid is an outspoken nonsense. There is nothing hybrid in EIGRP fundamental operation: it distributes vectors (i.e. arrays) of distances to all known networks. There is no topological detail being advertised in EIGRP (who is connected to whom and how). This is pure distance-vector approach, identical to RIP or IGRP. There are several additions to EIGRP to prevent forming of routing loops, to maintain adjacencies, to be event-driven instead of sending routing information in periodic intervals but all these are merely improvements to the basic distance-vector paradigm that has not been altered in any way in EIGRP. The EIGRP, as plain and blunt as it may sound, is just an advanced distance-vector routing protocol.
Okay, EIGRP Hello messages: I have attached a Wireshark-readable capture file that contains a number of EIGRP Hello packets. Have a look into them closely using Wireshark, the fields are mostly self-explanatory:
Version: currently, version 2 is used
Opcode: type of the EIGRP packet. 5 means Hello
Checksum: verifies the packet contents
Flags: Used to describe special treatment for this packet or its sender. Currently, 4 bits are defined: Init, Conditional Receive, Restart, End Of Table
Sequence and Acknowledge: used for acknowledged packets only (Update, Query, Reply)
Autonomous system: the AS of the router that sent this message
EIGRP Parameters: a set of Type-Length-Value triplets to carry additional information. In the Hello Packet, the K-values and Hold time are advertised here.
Software version: additional information about the IOS version
Does the update message containing prefix, prefix lenght, metrics and non metrics carry into hello message?
No, it does not. In EIGRP, the routing information (prefix, mask, metrics, additional information) is never carried in Hello packets. Hello packets are used only to detect neighbors and verify their continous presence on the segment, but Hellos are never used to carry the routing information itself. Routing information is carried in EIGRP Update packets. Also, information from Queries and Replies may influence routing table contents but the main packet type used to carry routing information in EIGRP is the Update packet.
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