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

["Alex P. Rudnev" <alex () Relcom EU net>]

> > 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?
>
>
> No.  The ACL specifies an exact match.
You should change 'host 255.255.0.0' to '255.255.0.0 0.0.255.255' to 
allow any _more specific_ mask. Read CISCO docs.



> > 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
>
> Same deal.
>
> Chris

Aleksei Roudnev, Network Operations Center, Relcom, Moscow
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