If two routers have two links between them and they have the same metrics assigned by what ever routing protocol is in use, both routes end up in the FIB with the same administrative distance. So, as far as I can tell CEF will use both routes at the same time to transfer data.
My question is this;
If I change the metric on one link so only one is used at a time, and the other becomes a backup route, i.e. if the first link goes down, the second link will be pushed to the FIB. But, my understanding is that changing the route to remove one link from production will instantly drop all traffic on that link, as it will be dropped from the FIB. Is there a way to gracefully "phase out" a route, rather than having all current flows dropped, interrupting traffic?