all the mails on Filtering

Jeff S Wheeler jsw at
Wed Nov 13 19:04:37 UTC 2002

On Wed, 2002-11-13 at 13:25, Harsha Narayan wrote:
>   So what happens to multihoming assignments made by the ISP? That means
> the multihoming assignment can't be used as a backup. If the customer's
This has been rehashed time and time again on this list, and others. 
The fact is, ISPs have to filter somewhere to keep routing table growth
in check.  It makes more sense to filter out announcements longer than
the longest assignment within an RIR's space than to filter on any
arbitrary boundary.  I think everyone will agree with that, even if they
do not agree that filtering is necessary or good.

As an example of why filtering is good, click on this link and visit the
CIDR Report AS Summary for an ISP here in Kentucky.  They used to have
over 50 useless announcements within one /18, for which they had an
aggregate announced as well.  They seem to have gone to some efforts to
reduce the route table pollution the emit, and I applaud their efforts,
however you can further reduce the amount of pollution you accept from
them, and other ISPs who mistakenly announce from their IGP or for any
other reason do not announce blocks as they are assigned by the RIR,
simply by filtering on the minimum assignment size.

I think there are very few networks who purposely announce longer
networks to control their inbound traffic flow, verses the number who
mistakenly do so.  Again, everyone will agree.  Except, perhaps, Ralph
Doncaster.  And if you want to spend your FIB entries, and your money,
making your bits flow to him in the manner that's most cost-effective
for ISTOP, then more power to you.  Most folks will agree that is up to
Ralph and Ralph's ISP(s) to work out, though.

Jeff S Wheeler <jsw at>

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