I am not an expert on this, I just wanted to document a problem I had and a solution I found today, in a concise way. Comments correcting me or suggesting better ways are very welcome.
I have a network running OSPF internally, and advertising routes to the upstream ISP over BGP at two separate edge routers (multi-homed, single ISP). We discovered last night that internally bringing down any of the subnets we advertise results in the dropping of those routes from the tables of the edge routers (as expected). This drops the advertisements. What we did NOT expect was that flap damping from upstream of us then null-routes that subnet for up to a few hours.
So, how do we retain our adaptive internal routing (OSPF) while avoiding route flap? I was a bit stumped about this, but I found a more complex article that describes a multi-homed BGP setup. A key part of that setup was a little trick to avoid this problem. Nameley, set up a static, black hole route for the subnet on the edge router, with maximum distance. This way, even if the OSPF route disappears, the router still “knows” a route to the subnet and won’t drop the advertisement.
For example, if you want to advertise the subnet 188.8.131.52/24, you should add a static route like
/ip route add dst-address=184.108.40.206/24 type=blackhole distance=254 comment="prevent flapping of the route over BGP"
I’ve tested it and it seems to work as expected. The route is not active as long as the OSPF route is in the routing table. If it disappears, the black hole route becomes active.