From: Carlos G Mendioroz (tron@xxxxxxxxxxx)
Date: Mon Jun 17 2002 - 16:29:47 GMT-3
Howard,
1) I think "no export" would not qualify as a valid solution because
R1 would not be advertising the route, as I understand the problem
statement requires.
2) I thought that prepend was just a way of tweeking the PATH attribute
as it is sent to a neighbour. How can a neighbour know if you did
some tricks on it or not ?
Thanks in advance for any insight,
-Carlos
"Howard C. Berkowitz" wrote:
>
> I'm copying this to the list so others can benefit.
>
> >this is a question for preparing the lab. I couldn't find how can I
> >solve.
> >But Bhisham's answer agrees with all the rules.
> >if you prepend R2's AS to the 10.10.20.0 route
> >R2 won't take it because of the BGP process rules.
>
> The point is that you are describing something that the NO-EXPORT
> community is specifically intended to solve.
>
> AS path prepending is not part of the current BGP standard, and
> indeed there is a Cisco knob to disable it in the decision process.
> It will be part of the new draft standard, but there are definitely
> implementations that won't understand it -- especially Cisco or
> others that are configured to follow the strict IETF algorithm.
>
> If your topology changes and the second AS is no longer adjacent, AS
> path prepending may not work any longer, so you've created a
> potential long-term maintenance vulnerability.
>
> Again, this is a case of using best current practice in the real
> world, rather than using any protocol feature that seems to meet the
> need. I really would find it hard to believe Cisco would prefer AS
> path prepending to NO-EXPORT, because NO-EXPORT is a supported Cisco
> feature.
>
> >Best Regards
> >umit
> >
> >-----Original Message-----
> >From: Howard C. Berkowitz [mailto:hcb@gettcomm.com]
> >Sent: Monday, June 17, 2002 20:31
> >To: Ccie (E-mail)
> >Subject: Re: BGP no-filter
> >
> >
> >At 3:59 PM +0300 6/17/02, \mit As8kan (TK-Network Gvz|mleri) wrote:
> >>Hi,
> >>
> >>I have got a problem like that :
> >>
> >>network 10.10.10.0 R1-------EBGP--------R2
> >>network 10.10.20.0
> >>
> >>I want to see only 10.10.10.0 at R2 bgp table. But if you look to the
> >>R1's advertised routes to R2, you will see 10.10.10.0 and 10.10.20.0.
> >>You can't do anything on R2 for that routes.
> >>
> >
> >I'm assuming R1 learns 10.10.20.0 through an IGP, static route, or
> >direct connection. Is there some reason that you can't tag it with
> >the well-known BGP community, NO-EXPORT, when it enters BGP? The
> >purpose of that community is to keep the route inside your AS.
> >--
> >"What Problem are you trying to solve?"
> >***send Cisco questions to the list, so all can benefit -- not
> >directly to me***
> >************************************************************************
> >********
> >Howard C. Berkowitz hcb@gettcomm.com
> >Chief Technology Officer, GettLab/Gett Communications
> >http://www.gettlabs.com
> >Technical Director, CertificationZone.com
> >http://www.certificationzone.com
> >"retired" Certified Cisco Systems Instructor (CID) #93005
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