To understand the possible use of this, consider a site where there are multiple Ethernet VLANs in use, perhaps dozens, but the administrator wishes to use a single /64 for all of them without bridging them together. This is conceptually similar to OSI's concept of "area" addressing.
Obviously, the routers would need to allow assigning the same prefix to multiple interfaces, and the L bit in the ICMPv6 RA's Prefix Information option would need to be clear (0). When L=0, all hosts on any of the VLANs would forward packets to new destinations to a router. If the destination were on the same VLAN the router would send a Redirect message; if not, the packet would be forwarded as normal. This looks fairly useful in some scenarios.
A few obvious problems come to mind:1. Any router connected to one VLAN in the "area" must be connected to all such VLANs. If not, routing protocol modifications would be required so that routers would be able to forward to destinations not on an attached link. 2. Multicast semantics are not obvious. Should a multicast packet of link scope be forwarded to the other VLANs? ICMP RA/RS/ND gets ugly if you do that. I think multicast packets of greater scope will work correctly, but I get lost in all the corner cases.
3. Hosts may not expect L=0 on Ethernets and behave incorrectly. Solutions? Other problems? Doubts about my mental health? S Stephen Sprunk "Stupid people surround themselves with smart CCIE #3723 people. Smart people surround themselves with K5SSS smart people who disagree with them." --Aaron Sorkin
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