When a adjacency falls down, the primary routes are not deleted on the
dataplane until the SPF is recomputed. Even the backup routes are
pre-installed on the dataplane, there is no fast-route optimization.
Reasons for an adjacency to come down are:
- BFD down
- Hello timer timeout
- User adjacency clear
Apply the backup route switchover for fast-reroute as soon an IS-IS
adjacency falls down before the first SPF re-computation. Pre-computed
backup routes are applied sooner.
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>