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  • Victor Mukhanov thinks this is interesting:

Although multiple OSPFv3 processes can run on a router, only a single process or instance can run on an interface.


Cover of Routing TCP/IP, Volume 1, 2/e


How come?
In the answer to the review question #2:
Multiple instance per link means that there can be separate adjacencies among different routers all connected to the same broadcast link, so that different OSPFv3 routing domains can use the same shared link without interfering with or having knowledge of each other. The Instance ID field in the OSPFv3 header makes this possible.