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Re: Upstreams blocking /24s? (was Re: How Not to Multihome)

  • From: Steven M. Bellovin
  • Date: Mon Oct 08 19:45:06 2007

On Mon, 8 Oct 2007 16:06:52 -0700
David Conrad <[email protected]> wrote:

> Hi,
> On Oct 8, 2007, at 2:48 PM, Scott Weeks wrote:
> > However, if it's less than a /24 it won't get very far as most >
> > upstreams block prefixes longer than a /24.
> I'm curious: a couple of people have indicated they do not believe
> this to be the case. Anybody have any hard data on what filters are
> actually in use today?

That's a good question. says
what was in use 6.5 years ago; it would be good to look at newer data.
> Others have indicated that such filters (assuming they exist) will
> not last in the face of paying customers presenting longer than /24
> prefixes for routing.  Specifically, that ISPs will relax their
> filters (allowing longer than /24) in order to get their peers to
> accept their long prefixes.  Anybody have an opinion on the
> likelihood of this?
The traditional answer has been "paying whom?"  A given ISP's customers
might pay it to announce their routes; *maybe* they'll have bilateral
agreements with some of their peers to carry each other's longer
routes.  But what about the next hop?

Put another way, there's been a lot of discussion -- pardon me, a
*FLEEPING LOT of DISCUSSION* -- on this list lately about how lots of
folks need to upgrade line cards and/or IOS and/or routers to keep up
with the growth of the routing table.  If the growth is due to long
prefixes, who pays?  Again, it's (relatively) easy to charge your own

		--Steve Bellovin,