bgpd: conditional default-originate using route-map

Incorporate a patch by Svetozar Mihailov which implements
default-originate route-maps to behave as expected, i.e. allowing
the default route to be advertised conditionally, depending on a
criterion given by the route-map.

I am aware that the performance attributes of the following implementation
are far from optimal. However, this affects only code paths belonging to
a feature that is broken without this patch, therefore, it seems reasonable
to me to have this in the mainline for now.

Cc: Svetozar Mihailov <quagga@j.zarhi.com>
Reported-by: Sébastien Cramatte <scramatte@gmail.com>
Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
3 files changed