Hello Luca
IS-IS uses a hierarchical structure using areas in a similar manner to how OSPF uses areas. Whenever you use areas, you separate the IS-IS topology into logical units that operate independently but are connected using Level 2 routers.
The redistribute isis ip level-1 into level-2 command is used to redistribute L1 routes into L2 in an IS-IS network. Similarly, the redistribute isis ip level-2 into level-1 command is used to redistribute routes back from L2 to L1. These commands are typically used on L1-L2 routers, which are boundary routers between L1 and L2 areas for the purpose of redistributing routes between IS-IS areas. In other words, redistribution is used to inject routes in one IS-IS area into another.
Now if you have only L1 routers, that means you only have a single area, which in turn means that all routes are already exchanged between routers through the normal IS-IS routing protocol operation. Thus, there’s no need to redistribute routes within the same level because they are already known to each other.
For this reason, the redistribute isis ip level-1 into level-1 is not valid. The Cisco command reference for this command states that you must:
Specify either level-l into level-2 or level-2 into level-1
I hope this has been helpful!
Laz