RE: BGP Filtering

["Ben Butler" <ben.butler () c2internet net>]

Hi Dave,
 
Yes that is what I was thinking I want to do - so I am guessing here - I
think what we are saying is the /17s never get re-added when the /16 is
withdrawn because this does not - for very good reasons when I think
about it- cause the filter to be evaluated upon the withdrawal of a
prefix, only on when it is newly announced does it get checked - or
maybe the odd table scan in the code?? But basically the /17s just sit
there and continue to be filtered.  Is that approximately correct?
 
so umm, yes a default would be needed, ummm.
 
Is it even technically possible to easily achieve though?
 
Ben

________________________________

From: Dave Israel [mailto:davei () otd com] 
Sent: 15 January 2008 17:51
To: Ben Butler
Cc: nanog () merit edu
Subject: Re: BGP Filtering



Ben,

  I think I understand what you want, and you don't want it.  If you
receive a route for, say, 204.91.0.0/16,  204.91.0.0/17, and
204.91.128.0/17, you want to drop the /17s and just care about the /16.
But a change in topology does not generally result in a complete update
of the BGP table.  Route changes result in route adds and draws, not a
flood event.  So if you forgot about the /17s and just kept the /16, and
the /16 was subsequently withdrawn, your router would not magically
remember that it had /17s to route to as well.  You'd drop traffic,
unless you had a default, in which case you'd just route it
suboptimally.

-Dave


Ben Butler wrote: 

        Hi,
        
        Agreed that is why I have lots of RAM - doesn't mean I should
carry on
        upgrading my tower of babble though to make it ever higher and
higher if
        there is a better way of doing things.
        
        I still don't see how a default route to a portioned pop is
going to
        help in the slightest - you are saved by getting the prefixes
from an
        alternate transit and the default doesn't get used.  Where is
does help
        is to capture anything which has been filtered out completely
and then
        there is no prefix from the alternate transit provider anyway -
so
        whichever default gets used and takes its chances.
        
        Bogons - obviously.
        
        My question was if what I was asking was possible.
        
        Kind Regards
        
        Ben
        
        
        -----Original Message-----
        From: Joe Abley [mailto:jabley () ca afilias info] 
        Sent: 15 January 2008 17:07
        To: Ben Butler
        Cc: nanog () merit edu
        Subject: Re: BGP Filtering
        
        
        On 15-Jan-2008, at 11:40, Ben Butler wrote:
        
          

                Defaults wont work because a routing decision has to be
made, my 
                transit originating a default or me pointing a default
at them does 
                not guarantee the reachability of all prefixes..
                    

        
        Taking a table that won't fit in RAM similarly won't guarantee
        reachability of anything :-)
        
        Filter on assignment boundaries and supplement with a default.
That
        ought to mean that you have a reasonable shot at surviving
de-peering/
        partitioning events, and the defaults will pick up the slack in
the
        event that you don't.
        
        For extra credit, supplement with a bunch of null routes for
bogons so
        packets with bogon destination addresses don't leave your
network, and
        maybe make exceptions for "golden prefixes".
        
          

                I am struggling to see a defensible position for why
just shy of 50% 
                of all routes appears to be mostly comprised of
de-aggregated routes 
                when aggregation is one of the aims RIRs make the LIRs
strive to 
                achieve.  If we cant clean the mess up because there is
no incentive 
                than cant I simply ignore the duplicates.
                    

        
        You can search the archives I'm sure for more detailed
discussion of
        this. However, you can't necessarily always attribute the
presence of
        covered prefixes to incompetence.
        
        
        Joe