BGP prefix filter list

William Herrin bill at
Mon May 20 22:05:45 UTC 2019

On Fri, May 17, 2019 at 9:06 AM Baldur Norddahl <baldur.norddahl at>

> Think about this way to save at least half the size of the FIB with two
> transit providers: Find out which provider has the most prefixes going
> their way. Make a default to them and a route-map that drops every route.
> For the other provider, keep only the routes where they have better
> routing. This way you only use FIB space for the smaller provider.
> Everything else goes by default through the larger provider.

Hi Baldur,

The technique you describe was one variant of FIB Compression. It got some
attention around 8 years ago on the IRTF Routing Research Group and some
more attention about 5 years ago when several researchers fleshed out the
possible algorithms and projected gains. As I recall they found a 30% to
60% reduction in FIB use depending on which algorithm was chosen, how many
peers you had, etc.

As far as I know there are no production implementations. Likely the extra
complexity needed to process RIB updates in to FIB updates outweighs the
cost of simply adding more TCAM. Another down side is that you lose the
implicit discard default route, which means that routing loops become

Bill Herrin

William Herrin
bill at
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