Cisco Route filtering [was Re: OPS: BGP spew from ASN 7374]

alex at tempest.nac.net alex at tempest.nac.net
Thu Apr 8 08:14:29 UTC 1999


I've had a question on this for some time that maybe can get answered
here.

Let's assume for a moment that I have a network of 209.123/18 (1/4 of a
classful B).

Let's further assume that my upstream filters me with:

access-list 2002 permit ip host 209.123.0.0 host 255.255.0.0
neigh 2.3.4.5 distrib 2002 in

Will my /18 announcement make it through this /16 filter?

Would the same be true with a prefix-length filter, which I assume the
syntax would be:

ip prefix-list 1 permit 209.123.0.0/16 
neigh 2.3.4.5 prefix-list 1 in


Thanks.




On Wed, 7 Apr 1999, R.P. Aditya wrote:

> 
> This should probably be moved to cisco-nsp if it gets any more in-depth,
> however:
> 
> > Cisco: could we please have more config memory, so we can load access-lists
> > for even huge BGP peers?
> 
> Although you're probably asking for exactly what you say, config memory, it
> might also be useful to use prefix-lists to filter routing announcements:
> 
>   http://www.academ.com/nanog/june1998/turner/sld014.htm
> 
> and the notes on bug CSCdk93005 seem to indicate that prefix-lists do make a
> difference and should be used over access-lists for filtering routes...
> 
> Hope that helps,
> Adi
> 
> 

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