From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Mon Jun 17 2002 - 15:34:44 GMT-3
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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