Daisy Chain Orbi Routers w/ IPv6?

While I looked briefly at getting double-routing ipv6 (Verizon-> Orbi 870), I dropped looking into it as other priorities came along.  IPv6 is currently disabled (and re-enabling and experimenting again would require several reboots of the router, which I don't want to do right now).

There are a couple basics:

Every device using IPv6 natively over the internet has a globally unique IPv6 address.  There is no NAT and no need for port forwarding. 

There is a large "link local" (not internet routable) address block. While these addresses can be used for local network data flows, the main purposes are more administrative. For instance allowing devices to discover other devices on the local network, get gateway information from their routers, etc.  So these are used differently than private IPv4 addresses.  These link-local addresses are always self-assigned. 

Also there is a special address block that is used to represent any ipv4 address as an ipv6 address.

Your ISP is giving you an address block of IPv6 addresses, not just a single IPv6 address. You do need to know how large that block is.  Verizon gives me a /56 block (which is usually the smallest block assigned to a single customer site, though some might give out a /60 block).  This is a huge address block - 2**72 globally unique ipv6 addresses.  

Normal practice would be to assign a /64 block (2**64 addresses) to the RBS750 clients.  (You don't want to go smaller than that, since processes like SLAAC break if you do).  That assignment is what the "Prefix Delegation" is about.  This is similar to the subnet concept in IPv4 (but again with no NAT).  The "Prefix" is the beginning section of the IPv6 address block (the part that is the same for all IPv6 addresses in the block).

So a /64 network has a 64 bit network prefix, followed by a 64 bit host section.  A /56 network has a 56 bit prefix, followed by 8 additional bits that you use to allocate up to 256 /64 subnets.  Then a 64 bit host section for each of those subnets.  Similarly, a /60 address block has a 60 bit prefix, followed by an additional 4 bits that you use to allocate up to 16 /64 subnets.

The Verizon router by default allocates the first /64 block for its local devices (subnet 0).  It has a field where I enter the subnet ID for its address distribution, so I can change that.  I can also set up the router explicity to use DHCPv6-PD, or to use SLAAC.

Not sure how to set up the Orbi to automatically take the second subnet for its address distribution - I didn't get that far in my investigation.  In my brief look, I found the Orbi set up to be quite confusing - the settings and terminology didn't map well onto my IPv6 terminology.  Although I'm not a huge fan of the Verizon web interface, its IPv6 settings are much clearer.