One more question I forgot to include: I see this message in the router's logs. Is it just a notice, or indicative of a problem?

Prefix Delegation Size incompatible with ISP: Dynamic IPv6 acquirement succeeded. (IPv6=<null>, Gateway=<null>, DNS=<null>.)

How could that possibly be true? If subnetting was not intented to be supported, then the Prefix Delegation Server configuration would only allow a single entry for specifying a delegated prefix. But it was explicitly designed to allow multiple entries for delegated subnets, clearly indicating intent to allow multiple delegated prefixes. That means subnetting. After all, subnetting is the very foundation of IPv6 routing, so an IPv6 gateway that does not support subnetting would fail the most basic definition of the term.

My current configuration has 4 delegated prefixes, and each one has an ID that the configuration page allows to be specified based on the difference between the received prefix (/56) and the delegated prefix (/62). Therefore we know that the ER7406 is aware of the difference, and that the configuration tool is calculating the difference and explicitly offering those bits for selecting a subnet. That is intentional and it is supporting the feature you claim is not supported. The facts countradict your statement.

Since we seem to be unable to communicate clearly, I am including screenshots to explain precisely what I am trying to understand.

Here is my WAN configuration page (censored):

As you can see, I am requesting a /62 from my ISP. I have confirmed with my ISP that they received the PD request with a DUID that matches my 00:03:...:A2:C2, and they responded by providing a /56, which conforms with the relevant RFCs. The prefix provided is 2001:db8:1241:0900::/56.

Here is my LAN configuration page. You can see it has two sections. One is for the local LAN, which services hosts connected to the direct link. The other is for a feature labeled "Prefix Delegation Server". It is on the same page, but it is a separate configuration from the LAN itself.

My current experiment with subnetting has four distinct entries for the Prefix Delegation Server. The "Add" and "Delete" buttons allow creating an arbitrary number of distinct entries, so clearly this is by design and consistent with TP-Link's intended usage of this feature.

Here is the expanded LAN configuration page.

As you can see, it is allocating addresses for ordinary hosts connected to the LAN from the prefix provided by the ISP. However, it is using the /56 provided but when allowing me to specify the IPv6 Prefix ID, it assumes the provided prefix is exactly what was requested on the WAN page (/62), when the actual prefix is larger (/56). That is why it only allows me to select a value in the range 0-3 (2 bits), which it uses to calculate the 2001:db8:1241:0903::/64 address pool for the LAN.

Hosts connected to the LAN have full IPv6 connectivity, so that part is working fine.

Here is the page for editing one of the several entries in the Prefix Delegation Server.

You can clearly see which fields are editable, and which are not. It allows me to select the WAN that received a delegated prefix, and it correctly populates the 2001:db8:1241:0900::/56 prefix that my ISP provided. Every single entry in the Prefix Delegation Server is using this exact same delegated prefix, which is the expected behavior if this server is here to support subnetting as is normal for IPv6 networks that consist of more than one physical link. Furthermore, the Prefix ID that it requires me to enter must take a value in the range 0-63, which is consistent with the 6 bits available between the /56 that the ISP provided and the /62 that the entry is configuring to be offered to routers (not hosts, routers) that exist on the LAN. Put simply, the only logical conclusion is that this is for subnetting, and child routers on the LAN will fetch prefixes from the ER7406 and use them for child links (such as wireless). Notice that the New Prefix (not editable since it is calculated by the ER7406) puts the Prefix ID entered in the six bits not covered by the /62, so the entries have values such as 2001:db8:1241:0904::/62 and 2001:db8:1241:0908::/62, etc. Furthermore, when I set my WAN to request a /56 and then add an entry to the Prefix Delegation Server on the LAN page, the only value allowed for the Prefix ID (in the delegation server entry) is 0, and the New Prefix ends up being 2001:db8:1241:0900::/56.

What is not explained is what I need to enter for the user-editable Link-local Address and DUID. They are required to have values, but there is no explanation for what the values should be. The Link-local Address does accept the value "::", but since I have not yet gotten my wireless router to solicit and receive a prefix from my ER7406, I have no way to test whether this value is correct or not. I have tried entering the DUID from my wireless router in the DUID field, and I have tried making up new values, but so far none of my experiments have yielded positive results.

Questions

What is the intended purpose of the Prefix Delegation Server? It is included as a standard feature of the router, how is it intended to be used?

Overlap?

What happens if I configure an entry in the Prefix Delegation Server that uses the Prefix ID of 0, which overlaps with the address space of the LAN? I am guessing addresses in the conflicting range would become unroutable. (This is why my entries begin with Prefix ID of 1, keeping the delegated subnets distinct from the LAN link.)

Link-local Address?

What value or values are acceptable for a delegated subnet's Link-local Address? Does the value "::" mean something special, such as to auto-populate the MAC address of the LAN interface on the ER7406? Do I need to manually enter in the LAN link-local address, beginning with FE80::...?

DUID?

What values are acceptable for the DUID of an entry in the prefix delegation server? Should individual entries match the DUID of child routers (such as a wireless router)? Should they be new, unique values? The configuration page says that the entered values can be as short as 2 or as long as 256 hexadecimal numbers, but if they need to match DUIDs received in soliciation requests, then they should be of the same length as standard DUIDs, right?

Prefix Length?

Why does the Prefix Delegation Server have no way to specify the Prefix Length of entries? Right now it simply uses the value as entered for the WAN, which seems like a bug. If the PD server in the ER7406 is intended to only offer the exact prefix received on the WAN interface, then it would not be possible to configure more than one entry for the Prefix Delegation Server. Since the interface allows having multiple entries (as demonstrated by the bright green "Add" button), the only logical conclusion is that this is designed for subnetting, which means entries here should always have a longer prefix than the one requested by the WAN.

Please help.

Clive_A wrote

If you use the Omada gear as the only PD in your IPv6 network, then the LAN config should be SLAAC+Stateless DHCP mode. 

If you need it to work, you set it to Get from Prefix Delegation so it gets the PD from the ISP and further subnets and distributes the IPv6 IP.

This is what I am trying to do. How do I control the behavior of the part you call "futher subnets and distributes the IPv6 IP"?

Clive_A wrote

I don't think we need to fill out the DUID on our devices.

 

I am not seeing that Link Local is required on our device when configuring IPv6. 

I have shown screenshots demonstrating that DUID and Link-local Address are required to be filled out. You can confirm this yourself using the emulators on this very website.

What values should they be set to?

  @Clive_A, I am clearly using standalone mode, there is no controller. I have shown screenshots demonstrating that there is a prefix delegation server, and explained that the only possible purpose of that server is to support the feature you claim is not supported.

Is your favorite color black? Is there a human that I can talk to?