IPV6 - SLAAC, DHCPv6 Stateless, DHCPv6 Stateful and M vs O bit

Question

Good day everyone,

I have questions regarding IPv6.

• (1) I implemented Stateful DHCPv6 and the messages went thru (SARR) and clients are getting ip addresses and dhcpv6 options, no problem! Now, I read somewhere that when you are doing this setup, you need to set the M bit flag and this command "ipv6 nd managed-config-flag" should be set in the router so during the RA there will be M bit flag set to 1... But I didn't enable this on the router and everything looks good... Is it mandatory or is it just a polite way to say to the clients that "ok, I see the M bit set to 1 then I need to use DHCP to make a DHCP request"

I am not really sure if I really need to update the router config with the M bit flag...

• (2) SLAAC - I tested this and I was getting the EUI-64 address no problem, but does SLAAC doesn't offer DNS just plain ipv6/link-local gateway of the router? For me, it doesn't makes sense when there is no DNS... I found that I can set the RDNSS from the router and the client was able to get a DNS which I feel happy about it...

• (3) Stateless DHCPv6 - This will still use SLAAC but then it needs additional parameters like DNS... I read somewhere that if you are doing this, you need to set the O bit flag...

So If I enable the O bit flag in the router without configuring the RDNSS and without having a DHCP Pool in the router, How does a client gets the DHCP Option like DNS? Can this be combined with DHCP Relay??

Many Thanks!

Answer 1

The M and O flags are mostly advisory. Some operating systems don't try DHCPv6 unless those flags are set, some try anyway regardless of the flags, and some never try DHCPv6 at all (looking at you Android)