When the kernel reports the deletion of an interface (RTM_DELLINK),
the cached interface attributes, including ifindex, become invalid
and must be forgotten.
Interface link state changes ("up" and "down") show up as RTM_NEWLINK,
so they will not cause a cached entry to be removed or
prevent listening to address change notifications.
Once an interface has been deleted, the kernel ought to stop sending
notifications for it. If the interface gets recreated with the same
name later, the kernel again reports RTM_NEWLINK, which causes a new
cache entry to be created.
There should be no reason to keep a stale cache entry around, as was
claimed in the comment.
starter fails to load a connection when a peer's DNS name is temporarily
unresolvable and keyexchange=ike was specified, which defaults to IKEv2.
The connection loads just fine in case of keyexchange=ikev2.
Mirko Parthey