BGP Filtering

Tue Jan 15 22:08:52 UTC 2008

> But if I can see the /19 in the table, do I care about a load of /24s
> because the whole of the /19 should be reachable as the origin AS is
> announcing it somewhere in their network and it is being received my a
> transit so should be reachable.

The "presumption" in cases like this is that the /24 may take a 
different path than the /19 in some or all cases. If you have only a 
single provider you can safely dump more specifics -- but then, you 
could just point default. If you *are* multihomed and the /19 and /24 
both have the same primacy (first choice in a routing decision and same 
path) you can safely drop the more specific.

The "presumption" is that in some cases the /24 would take a different 
path than the /19 in a routing fight.

How much cost you want to incur for these is your choice. If enough 
people drop the more specifics, they will go away as well -- if they 
provided no benefit, fewer would exist.

Some of this originates from the peering-contests where folks have "x 
number of prefixes" which makes them bigger than "y number of prefixes".

I'd be interested to see any metrics on rate of growth of allocations 
longer than RIR limits since Verio instituted then dropped mandatory 
prefix filters. (vs the rate of growth of prefixes overall). I would 
guess that they accelerated.

Deepak