BGP prefix filter list

Baldur Norddahl baldur.norddahl at
Fri May 17 20:02:45 UTC 2019

On Fri, May 17, 2019 at 9:44 PM Blake Hudson <blake at> wrote:

> Baldur, I believe most routing platforms already make use of clever
> shortcuts or techniques to reduce their FIB usage, but I don't think anyone
> has found a good, reliable method of reducing their RIB at zero cost. For
> example, what happens in your above configuration when your
> "better/default" transit provider is down due to maintenance or outage and
> your equipment continues to use its default route to direct traffic that
> direction?

You will of course have two default routes, one to each transit provider.
Using route priorities to program which one is actually used. If that link
goes down, that default becomes invalid and the router will use the other
one. A more advanced setup can use triggers, such as ping, bfd or BGP, to
mark the route as valid or invalid.

> What happens if the transit provider that you normally only retain the
> best paths for becomes the best path for all destinations (for example if
> your connection to the better/default transit provider is down for
> maintenance or there is an upsteam peering change) and your router that
> normally only has a few thousand routes in RIB suddenly gets tasked with a
> 768k-1M route RIB?

I am not sure I am following that question. Nothing happens, you will have
a default plus a bunch of redundant routes, but not any more than you had
before the primary transit went down.

> I would argue that one can generally safely add information to his or her
> router's RIB (such as adding a local preference, weight, or advertising
> with prepends to direct traffic toward a better performing, less utilized,
> or lower cost peer), but that removing information from a router's RIB
> always comes at some cost (and some may find this cost perfectly
> acceptable).
One needs to remember that removing information from RIB is how BGP works.
If you have the common setup of two BGP edge routers, each with a directly
connected transit provider link, the routers will only tell the other one
about the routes it actually uses. Neither router has a complete view.


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