From: Bob Chahal (bob.chahal@xxxxxxxxxxxx)
Date: Sat Jun 02 2001 - 12:58:21 GMT-3
Roman
just so I can develope a clearer understanding of these types of
access-lists wouldn't
access-list 101 deny ip 0.0.0.0 255.255.224.0 0.0.0.0 255.255.224.0
do the same thing as
access-list 100 permit ip 0.0.0.0 255.255.224.0 255.255.224.0 225.255.224.0
Bob
----- Original Message -----
From: "Roman Rodichev" <rodic000@hotmail.com>
To: <bob.chahal@ntlworld.com>; <dward@pla.net.au>; <ccielab@groupstudy.com>
Sent: Saturday, June 02, 2001 4:19 PM
Subject: Re: Match prefixes gt using access-list?
>
> Actually. That's right, it depends on what you mean Darren. Is "better"
> longer or shorter? "Better" could be a more specific route. But "Better"
> could be a larger summary. It's just I can't find a reson why you would
want
> to get >/19 routes. ISPs usually want larger summaries. That's why this
> should be correct:
>
> access-list 100 permit ip 0.0.0.0 255.255.224.0 255.255.224.0
225.255.224.0
>
> You basically want the least significant 13 bits (last 13 bits) of the
mask
> to be 0's. So these masks would match:
>
> 255.255.224.0
> 255.255.192.0
> 255.255.128.0 and so on...
>
> But this one wouldn't: 255.255.240.0
>
> my 2 cents
>
>
> >From: "Bob Chahal" <bob.chahal@ntlworld.com>
> >Reply-To: "Bob Chahal" <bob.chahal@ntlworld.com>
> >To: "Darren Ward" <dward@pla.net.au>, <ccielab@groupstudy.com>
> >Subject: Re: Match prefixes gt using access-list?
> >Date: Sat, 2 Jun 2001 12:35:09 +0100
> >
> >No sure what you mean by better but if you want to filter /19 or longer
> >prefixes from a neighbor
> >
> >then
> >
> >access-list 101 deny ip 0.0.0.0 255.255.255.255 255.255.224.0 0.0.31.255
> >access-list 101 permit ip any any
> >
> >router bgp xx
> >neighb y.y.y.y remote zz
> >neighb distribute-list 101 in
> >
> >
> >
> >----- Original Message -----
> >From: "Darren Ward" <dward@pla.net.au>
> >To: <ccielab@groupstudy.com>
> >Sent: Saturday, June 02, 2001 11:16 AM
> >Subject: BGP: Match prefixes gt using access-list?
> >
> >
> > > Hi All,
> > >
> > > I've ben asked a damn good question by a co-worker and I must admit
I'm
> > > not exactly sure how to solve it.
> > >
> > > Q: Filter BGP advertisements to a neighbour so that only routes that
are
> > > /19 or better are advertised WITHOUT using a prefix-list.
> > >
> > > A:
> > >
> > > I know it will be a BGP extended IP access-list using the network/mask
> > > pair but can't figure out the mask section properly.
> > >
> > > access-list 100 permit ip 0.0.0.0 255.255.224.0 255.255.224.0
> > > 225.255.224.0
> > >
> > > Now, I _think_ this means match ANY network where the last 13 bits are
> > > all zero AND the last 13 bits of the netmask are all zeros but allow
> > > more zeros the base 13 bits (greater than /19)
> > >
> > > Is this correct?
> > >
> > > Darren Ward
> > > (Sydney June 27-28)
> > > **Please read:http://www.groupstudy.com/list/posting.html
> >**Please read:http://www.groupstudy.com/list/posting.html
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