Comment 0 for bug 1738219

When the 'ignore_routes_with_linkdown' sysctl is set, we should not
consider linkdown nexthops during route lookup.

While the code correctly verifies that the initially selected route
('match') has a carrier, it does not perform the same check in the
subsequent multipath selection, resulting in a potential packet loss.

In case the chosen route does not have a carrier and the sysctl is set,
choose the initially selected route.

Fixed in the upstream kernel with patch:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bbfcd77631573ac4a9f57eb6169e04256a111bc1

I think this patch must be backported to stable kernel in Xenial. I tested in patching the Ubuntu Xenial 4.4.0-21-generic and it works for me.