Routing - link-local IPv6 next hop address needs exit interface

When routing in an IPv6 environment, the next hop IPv6 address typically used by routing protocols such as OSPF and EIGRP is the IPv6 link-local address.

Similarly, when creating a static route with a next hop IPv6 address pointing to the link-local address, 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 we can use the same link-local address on all the interfaces of a router. Or, we 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.

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.

If there is only a next hop IP in a particular routing entry, a recursive routing lookup will take place with that IPv6 address to find the exit interface. Since the link-local addresses are not found within the routing table as destinations since they're not routable, the exit interface will not be found, the lookup will fail, and the packet will be dropped.