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The router for my local network advertises two IPv6 prefixes (one local, fdXX:: prefix, and one delegated from my ISP). I've confirmed that they're both being advertised by inspecting the router advertisements with Wireshark, thus:

router advertisement contents

However, Windows hosts on the network only seem to pick up one of the prefixes (and consistently the fdXX:: one) to use:

Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . : arkane-systems.lan
   IPv6 Address. . . . . . . . . . . : fdc9:b01a:9d26:0:fd4c:9cca:2051:c663
   Link-local IPv6 Address . . . . . : fe80::21cd:60d4:2e62:cc29%14
   IPv4 Address. . . . . . . . . . . : 172.16.1.2
   Subnet Mask . . . . . . . . . . . : 255.255.0.0
   Default Gateway . . . . . . . . . : fe80::217:94ff:fef2:c3f%14
                                       172.16.0.254

Why is this happening, and is there any way I can get the hosts to pick up on both interfaces?

Cerebrate
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    Both prefixes are wrong. SLAAC is only supposed to work with /64 prefixes. The puzzling part is that a prefix is used at all... – Sander Steffann Aug 18 '16 at 01:46

1 Answers1

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My first question I would ask is why you are announcing 2 prefixes. I am not sure how Windows built its stack, but I would expect it is looking for one route.

The fdc9: prefix should not be used for global (internet) routing. However, given the lifetimes, it appears to be the preferable network in terms of lifetime. Also it is closer to the desired /64 for a local routing block. Is there any reason for having a local identifier when the global identifier would work as well?

The 2600: prefix is being advertised with the full /48 supposedly supplied by your ISP. Normally this would be broken down into /64 sub-nets with each router being assigned a sub-net. This would be good for 65536 routers in your organization. Try advertising a /64 subnet and see what happens.

BillThor
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    Why you're announcing two prefixes? Because one is for global connectivity and one is ULA. There's nothing wrong with this. – Michael Hampton Aug 18 '16 at 04:36
  • Yep, exactly what @MichaelHampton said. I want ULAs alongside the ISP-assigned block for the benefit of those few irritating applications that demand hard-coded IP addresses, such that I won't have to reconfigure them should I ever change ISPs in the future. – Cerebrate Aug 18 '16 at 06:01
  • Also, thanks muchly - evidently sometime this week my brain slipped a cog and lost track of SLAAC's need for a /64 prefix. *headdesk*. So reconfiguring both prefixes to /64 subsets of the whole fixed everything nicely. – Cerebrate Aug 18 '16 at 06:04