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

Thu Apr 8 23:16:15 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?

No.  The ACL specifies an exact match.

> 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