Hello Andrew
The link local address is an IPv6 address that is automatically assigned to all IPv6 interfaces. It is a fundamental part of how IPv6 functions. You can find out more about it at this lesson:
The link-local address is assigned using various mechanisms, and those mechanisms depend on the vendor of the equipment or the operating system being used. Specifically, Cisco uses the EUI-64 method to determine the link-local address.
Now you bring up a very good point, because loopbacks don’t actually have a MAC address, not even a virtual one, so the EUI-64 method will not work directly. When you enable IPv6 on a loopback interface, it will actually “borrow” a MAC address from another interface and use it to determine its link local address. This isn’t actually a problem, because link local addresses must be unique on each network segment. In fact, you can have the same link local address on all of your routed interfaces on a network device without any problem.
The ipv6 enable command is only needed if you want to enable IPv6 on an interface that has not been configured with an IPv6 global unicast address. When an interface is explicitly configured with an IPv6 global unicast address, it automatically becomes IPv6 enabled, and there is no need to issue the ipv6 enable command. Interestingly, if you issue the no ipv6 enable command on an interface with a configured IPv6 address, the command would be ignored. Notice in the lesson, that the 2001::1 and 2001::2 addresses have been assigned to the loopbacks, so the ipv6 enable command is not necessary.
I hope this has been helpful!
Laz