- systemd-networkd and initscripts both support it.

- it seems suggested to configure routes with scope "link" on AWS.

- the scope is only supported for IPv4 routes. Kernel ignores the
  attribute for IPv6 routes.

- we don't support the aliases like "link" or "global". Instead
  only the numeric value is supported. This is different from
  systemd-networkd, which accepts names like "global" and "link",
  but no numerical values. I think restricting ourself only to
  the aliases unnecessarily limits what is possible on netlink.
  The alternative would be to allow aliases and numbers both,
  but that causes multiple ways to define something and has
  thus downsides. So, only numeric values.

- when setting rtm_scope to RT_SCOPE_NOWHERE (0, the default), kernel
  will coerce that to RT_SCOPE_LINK. This ambiguity of nowhere vs. link
  is a problem, but we don't do anything about it.

- The other problem is, that when deleting a route with scope RT_SCOPE_NOWHERE,
  this acts as a wild care and removes the first route that matches (given the
  other route attributes). That means, NetworkManager has no meaningful
  way to delete a route with scope zero, there is always the danger that
  we might delete the wrong route. But this is nothing new to this
  patch. The problem existed already previously, except that
  NetworkManager could only add routes with scope nowhere (i.e. link).