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Hello may (legally) occur situation when router is not sending router advertisements but is providing configuration via DHCPv6?

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  • I guess "legally" you could have a router configured a DHCPv6 server that is not the default gateway, so RAs would come from another device
    – Benjamin Dale
    Commented Sep 4, 2023 at 7:31

2 Answers 2

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According to RFC4862, Sec 5.5.2 this can happen. However:

  1. DHCPv6 does not have an option to provide address of default gateway (explained at the same reference), so in this case default gateway must be configured manually.
  2. if the router want to disable SLAAC and use DHCPv6 instead, the proper way would be to use a special bit in router advertisement, that disables SLAAC. To do this you set M flag (AdvManagedFlag) on the router advertisement and disable all A flags (AdvAutonomousFlag) on each prefix. See RFC4861 for the description of flags. More info here
  3. you might want to consider this link as well.
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Yes. If you have DHCPv6 you will still need to make sure that the router is doing RAs (router advertisements) and these are not blocked as this is the only way the client can know about gateways.

I am unable to disable SLAAC on my ubiquiti router. It makes me choose between SLAAC and DHCPv6 (i implemented the latter on 2x windows server 2022 DHCP server for redundancy and visibility). All seems to work. I am unclear how a client decided between using SLAAC or DHCPv6 though.

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