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RFC 2462 specifies:

5.5.2. Absence of Router Advertisements

If a link has no routers, a host MUST attempt to use stateful autoconfiguration to obtain addresses and other configuration information. An implementation MAY provide a way to disable the invocation of stateful autoconfiguration in this case, but the default SHOULD be enabled. From the perspective of autoconfiguration, a link has no routers if no Router Advertisements are received after having sent a small number of Router Solicitations as described in [DISCOVERY].

If I understand correctly is that if a host fails to receive a router advertisment upon solicitation or within specified time limit, it should use DHCPv6 for address configuration.

Since DHCPv6 cannot carry the information about default gateways (https://datatracker.ietf.org/doc/html/draft-droms-dhc-dhcpv6-default-router-00 is deprecated), what is the point of address configuration (the address cannot even be used for link-local traffic since DHCPv6 lacks prefix information)?

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The Neighbor Discover Protocols, of which Router Advertisements are a part, is currently the only way to auto-configure the router options for a client. It's being debated whether this is an actual shortcoming of the DHCPv6 protocol, or if NDP-RA is sufficient. There's more than a few people who contend DHCPv6 is wholly unnecessary and should be deprecated too (I wont wade into that discussion here, but it's "hot").

Also, the DHCPv6 Router Option draft you linked is the original draft, they're currently on draft 4.

The essential argument is that if your router(s) are not broadcasting NDP-RA then they must not be working correctly, and you wouldn't want a client trying to contact them anyway. This allows you to configure a backup router more easily as the clients would automatically find it.

The Windows (6.0+), BSD, and Linux clients I have tested will all accept NDP-RA announcements and seek DHPCv6 information as one would expect. Currently you can auto-configure these clients for DNS with the mDNS/Bonjour protocol (an unacceptable solution for most enterprises). NTP has manycast and broadcast capabilities. WINS has NetBIOS. Only a few commonly used configuration options can't be replaced yet (notably PXE and the centralized management aspects of DHCP).

Neither DHCPv6 nor NDP address the security issues of computers receiving automatic configuration information over an essentially untrusted network. This is especially true of completely unprovisioned computers that have no certificates or information to base security on.

Chris S
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