Router Advertisements are enabled automatically for routing nodes
as soon as they obtain a prefix (e.g. via DHCPv6, UHCP, …).
Enabling router advertisements without a prefix to advertise is
wrong and causes no router solicitations to be send on an upstream
interface.
Also from https://www.rfc-editor.org/rfc/rfc4861#section-6.2.1
Note that AdvSendAdvertisements MUST be FALSE by
default so that a node will not accidentally start
acting as a router unless it is explicitly
configured by system management to send Router
Advertisements.
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