The /17 isn't sitting there still being filtered; it was never there to begin with. Your router heard the /17, saw that it didn't want it because of your filter settings, and promptly forgot it. You can tell your router to remember routes it doesn't install; it's called soft reconfiguration on a Cisco and is the normal mode of operation for a Juniper. But if you do that, you're not saving memory; an inactive route does not take less RAM than an active one.

I am pretty sure that there isn't a way to match a route on whether a larger aggregate exists using the current route map/policy statement verbage on the routers I have worked with. Doing so would be a reasonably simple code tweak, but without a purpose it isn't a tweak you're going to see any time soon.

-Dave

Ben Butler wrote:

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I want to do - so I am guessing here - I think what we are saying is the /17s never get re-added when the /16 is withdrawn because this does not - for very good reasons when I think about it- cause the filter to be evaluated upon the withdrawal of a prefix, only on when it is newly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 So if you forgot about the /17s and just kept the /16, and the /16 was subsequently withdrawn, your router would not magically remember that it had /17s to route to as well.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
  