What is the ICMPv6 redirect message and what does it do?

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Accepted Answer

An ICMPv6 redirect tells a host a better next-hop router for a given destination — the same function as the ICMPv4 redirect message. It is part of the Neighbor Discovery Protocol.
ICMPv6 redirect:
The redirect message is one of the messages included in ICMPv6 Neighbor Discovery (ND). A router uses it to inform a host of a more optimal first-hop router to reach a particular destination, so subsequent traffic for that destination is sent to the better router directly.
Key insight: This serves the same purpose as the redirect message in ICMPv4 — it improves routing efficiency on a segment where more than one router is available, without requiring the host to run a routing protocol.
Go deeper:

RFC 4861 — Neighbor Discovery for IP version 6 (IPv6) — its Redirect section explains how a router points a host at a better first hop.
 Neighbor Discovery Protocol — Wikipedia — redirect among the five NDP message types.