Windows 10 & IPv6 - RAs with set O flag lead to flag flapping (bug?)

Question

Hello,

this text has already been posted in the German Community forum. Since a nice Microsoft agent asked me to publish it in this forum, I am doing so now. I have just freely translated this post.

I am currently observing a very strange behavior of my Windows 10 clients while rolling out IPv6 in the network.

There is a Fritzbox 7590 in the network as a gateway with the latest firmware. The router obtains a public IPv6 subnet from the provider and advertises this via RA in the local network. The router is also configured with a ULA prefix for internal purposes. This is also advertised via RAs. I have currently deactivated the additional announcement of the local DNSv6 server via RA.

The router is also configured so that it only sets the O flag in the RA. The computers should therefore obtain the necessary information such as DNSv6 server or DNS prefix from the available DHCPv6 server via DHCPv6.

And this is where the problem begins.

The Windows computers configure their adapter correctly via SLAAC after they have received the RA from the router. However, they ignore the replies from the DHCPv6 server, which responds to the information requests. This means that no DNSv6 server is set for the adapter, despite the fact that the DHCPv6 sends a correct response.

BUT: not always. If IPv6 is deactivated and reactivated several times, the Windows client sometimes sets its DNSv6 server.

An ipconfig /renew6 also leads to "The following error occurred while updating the "Ethernet" interface: The time limit for the semaphore has been reached.".

I then checked the event display under DHCPv6 client. I noticed that 2 new events appear every time IPv6 is reactivated (freely translated):

The router announcement settings have been changed for network adapter 17. The current flag "M - Managed address configuration" is false, and the flag "O - Other stateful configuration" is true. User response: Frequent occurrence of this event may be due to frequent changes to the M and A flag settings of the router on the network. Contact the network administrator to resolve the problem.

Following (latest entry, freely translated):

The router advertisement settings have been changed for network adapter 17. The current "M - Managed address configuration" flag is true and the "O - Other stateful configuration" flag is true. User response: Frequent occurrence of this event may be due to frequent changes to the M and A flag settings of the router on the network. Contact the network administrator to resolve the problem.

You can see that the M flag switches back and forth between true and false. I then used Wireshark to check again whether the router's RA is being sent correctly without the M flag, which is the case. In addition, no other RA is sent from another device.

It rarely happens that the first event (the oldest) has the M flag set to true and the last event (the newest) to false. These are also the moments when the Windows client correctly processes the DHCPv6 replies.

Since I've been analyzing this for days and haven't come to any further conclusions, I'll try to get some help here.

It seems to me that I have stumbled across a bug here?

EDIT: I have just tried the whole thing with Windows 11 in a VM. The problem is not reproducible here. The flapping events do not occur here either. However, the same VM configuration leads to the described problem with Windows 10. So apparently it really is a bug in the IPv6 stack of Windows 10.

Answer 1

Hello,

 

Thank you for posting in Q&A forum.

As "M - Managed address configuration" is false, and the flag "O - Other stateful configuration" is true, let's try to run CMD command

netsh interface ipv6 set interface "Local Area Connection" managedaddress=enabled to enable managed address configuration.

Meanwhile you can check the router’s RA settings and ensure consistent flag configurations.

 

Best regards，

Jill Zhou

 

---

If the Answer is helpful, please click "Accept Answer" and upvote it.