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Re: BGP filtering of supernets out of classful space

  • From: Jeff Haas
  • Date: Fri May 19 12:57:13 2000

On Fri, May 19, 2000 at 12:31:20PM -0400, Daniel Senie wrote:
> Just how is the ISP in question going to contact each ISP doing this
> filtering? To be more specific, how will they know which ISPs ARE doing
> the filtering? Sure, they'll know about the ones they specifically have
> observed issues with, but beyond that, there's just no way to know.

No, there is no way to know.  For similar reasons, one can never know
if one's routes are going to be accepted by anyone.

One of my favorite examples, which I'm certain many providers here run
into while training individuals new to Internet routing, is the
transfer of Lore.  One can't just buy "Internet Core Routing, for
Dummies" and learn all of the tips and tricks which many NANOG
participants are aware of.  You wouldn't know that several providers
filter at /19 or /20, or may accept only classful announcements for
the old classful space, except except except....

All providers are welcome to provide links to their filtering policies
to the NANOG filter policies page at
However, I'd argue the right place to put it would be in the IRR.

> I continue to be concerned that some ISPs who do filtering do not
> provide ANY means to look at the routes they ARE accepting, nor
> necessarily post their filtering policy, thus providing very little
> useful mechanism other than complaints from end users for figuring out
> where problems exist. As a community, I think we can and should be
> working toward a much more robust mechanism.

For policy diagnostic purposes, the IRR is a fine place to put the
data.  A web page would suit as well but is hard to generate
filters from automatically.  Additionally, if its in a portable format
one doesn't have to translate it from provider A's router-config du jour
to your router-config du jour.

I can sympathize with ISPs that don't wish to provide looking
glasses into their networks.  The looking glass provides data that
several people in competing marketing departments would love - 
who they are peering with privately, internal topology views, etc.
Admittedly some of this can be filtered on the looking glass,
but why take the chance.

(And I'd like to thank those providers who do make looking glasses
available.  They make the Internet much more pleasant to troubleshoot.)

> The routing registries are only part of the solution. Even when the
> proper data is there, cases have existed where providers use that data
> plus or minus additional data to decide what to accept.

Similarly, one can't necessarily trust a given looking glass completely.
A failure in a full mesh, AS confederations or route reflectors can
result in an incomplete view.  But it gets you over that first hump
for the clueful when you need to call some network's routing engineers
to solve routing issues.

> Dan

Jeffrey Haas - Merit RSng project -

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