This document shows that you can use one of these options:
interface type/number
ipv6 address ipv6-prefix/prefix-length eui-64
ipv6 address ipv6-prefix/prefix-length link-local <------format
ipv6 address ipv6-prefix/prefix-length anycast
On the other hand when you try the command line options you get these:
R1(config-subif)# ipv6 address ?
WORD General prefix name
X:X:X:X::X IPv6 link-local address
X:X:X:X::X/<0-128> IPv6 prefix
autoconfig Obtain address using autoconfiguration
dhcp Obtain a ipv6 address using dhcp
and the command ipv6 address FE80::1 link-local will work without following the format pointed above. What am I missing here?
Edit: here are some tests using the format indicated by documentation
R1(config-subif)#ipv6 address FE80::1/11 link-local
^
% Invalid input detected at '^' marker.
R1(config-subif)#ipv6 address FE80::1/10 link-local
^
% Invalid input detected at '^' marker.
R1(config-subif)#ipv6 address FE80::0/10 link-local
^
% Invalid input detected at '^' marker.