Route table growth and hardware to the filter

Jon Lewis jlewis at
Sat Sep 8 04:38:20 UTC 2007

On Fri, 7 Sep 2007, Leo Bicknell wrote:

> The CIDR report states that we have 235647 routes that could be
> aggregated to 154503 routes.  While not the same metric, I'd be
> surprised at 147,564 routes if you did not have reachability issues.

If everyone behaved and announced their CIDRs as allocated (or even just 
deagged down to RIR minimum allocation size), those 147,564 routes would 
get you to everyone (in some cases suboptimally).  Obviously, anyone with 
PA-using BGP customers would need to punch some holes to allow those 
customer subnets through.

The trouble is, it turns out there are a number of networks where CIDR 
isn't spoken.  They get their IP space from their RIR, break it up into 
/24s, and announce those /24s (the ones they're using anyway) into BGP as 
/24s with no covering CIDR.

So, use of this prefix-list without a default route will cut off portions 
of the internet.

> One idea I've seen tossed around is to allow for a small amount of
> deaggregation.  For instance, if in a /8, the RIR allocates down
> to a /20, you might allow a /21 (break it into two blocks) or a /22
> (break it into four blocks).  Yes, that allows people with bigger
> allocations to break into more blocks, but it also allows everyone
> to do some TE without letting them do an unlimited amount.

I'm not crazy about that, but certainly it'd work, and there would still 
be some savings.  Due to the above mentioned stupidity, you'd still have 
no routes for some parts of the internet.

> I fear some filtering is in our future.  I'm not really opposed to it,
> either.  However I'm afraid your results show the currently available
> filters to be too aggressive.

If filtering is inevitible, I think it's worth reviving the CIDR police 
and perhaps scaring some clue into the networks that stand to be filtered 
off the net by anyone needing to do any level of filtering.

  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
_________ for PGP public key_________

More information about the NANOG mailing list