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Message 1 of 3

IPv6 Router Advertisement Details (SLAAC)?

I'm about to put my 3rd party router back in place after having my smart hub in place for a few days (50mbps cap still hasn't been lifted on 900 plan).

I've just run wireshark in the hope of capturing how BT set up their router advertisements so that I can set up my opnsense router in the same way.  From wireshark I can see:

Default Router Preference: Medium
Router lifetime: 180s
Valid lifetime: 300s
Preferred Lifetime: 120s
MTU: 1492

The two critical pieces of info I can't seem to find in wireshark are the min and max RA intervals which I believe from RFC2461 default to 200 and 600 sec respectively.  However BT must be using lower values for intervals as the lifetime values above are lower.

Any ideas what BT use?  If the interval values are set too low then some devices start showing weird behaviour such as not picking up ipv6 DNS servers via RDNSS.

Edit: what's interesting is my linux server is setting itself up with a valid lifetime of 258sec and preferred lifetime of 78 sec.  Still can't find the appropriate min and max interval.

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Message 2 of 3

Re: IPv6 Router Advertisement Details (SLAAC)?

I use pfSense and use the default DHCPv6 RA values and it all works fine on my network.  My Min RA interval is 200s and Max is 600s.  Router lifetime is the default of 1800s (3 x Max RA).

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BT FTTP 500/75 + pfSense + 4 x UniFi Wifi 6 Pro
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Message 3 of 3

Re: IPv6 Router Advertisement Details (SLAAC)?

Thanks for your reply.  With those values some of my android clients lose ipv6 infomation (GUA, gateway, ipv6 DNS etc) periodically either after sleeping or just instantaneously.  This doesn't happen with the BT smart hub in place and it looks like BT use lower values.

I'll put my opnsense router back in shortly (I'm still capped at 50 mbps upload with the SH) and reduce the values and try and find a good fit.  Trial and error here we come.

I have tried very low values in the past for min and max interval of around 5 and 20s respectively and when going that low I've noticed that android phones don't pick up ipv6 DNS via RDNSS for example.

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