Hello Patrick
In the lesson, Rene states the following:
When you use a global unicast address as the next hop, your router can look at the routing table and figure out what outgoing interface to use to reach this global unicast address. With link-local addresses, the router has no clue which outgoing interface to use so you will have to specify both the outgoing interface and the link-local address.
The reason for this is the fact that a link-local address must be unique only within the network segment it belongs to. That means I can use the same link-local address on all the interfaces of my router. Or, I can assign the same link-local address to all of the neighboring routers. So there is no guarantee that a particular link-local address will point to only one next-hop router. Take a look at the following diagram:
This is a valid setup with the same link-local address on the Fa0/0 interfaces of all three routers. So there is no way to uniquely identify a link-local address with a particular exit interface on R1 to reach such an address.
Secondly, link-local addresses are not routable, therefore, they don’t appear within the routing table as destinations. They do appear as next-hop addresses but never as destinations to reach.
Let’s say I had a routing table entry like so, without the exit interface. What would happen?
R1(config)#ipv6 route 2001:DB8:2:2::/64 FE80::41F0
Well, if R1 received a packet with a destination IPv6 address of 2001:DB8:2:2::1, it would look it up in the routing table and find this entry. It would then take a look at the next hop IP. It must, however, find the exit interface. How? It will do a recursive lookup of the FE80::41F0 address. However, it will not find it in the routing table. because link-local addresses are not routable. Therefore the packet will be dropped.
For all of these reasons, using the link-local address as the next hop without specifying an exit interface will fail. You must specify the exit interface along with the link-local next hop address.
I hope this has been helpful!
Laz