From: Hongbing Qin (hqin@houston.sns.slb.com)
Date: Mon Sep 16 2002 - 01:13:24 GMT-3
Brian,
When I reviewed this thread again, I find I still cannot figure out the
solution if we exactly follow the original requirement.
1, Weight is not an option, because it should be configured on R3, not on R2.
2. Local preference is not an option, because you are not allowed to touch
R3, and it's not possible to send Local Preference out.
3. AS-Path is not an option, because it should be configured on R1's
neighbor to R3.
4. Origin code is not an option, because the route is already there.
5. MED is no an option because R3 get the route from R1 without Metric
(NULL), so it's impossible for R2 to send lower metric to R3.
6. Obviously, R3 ebgp with R1, and ibgp with R2.
Am I right here? Please correct me if I am wrong. Any help? Thanks.
Best Regards,
Hongbing Qin
At 11:32 AM 8/25/2002 -0700, you wrote:
>Thai,
>
> The BGP decision process is:
>
>Weight
>Local-Preference/locally originated
>AS-Path
>Origin code
>MED
>EBGP over iBGP
>Shortest internal path
>Router-ID (lowest)
>
> Based on this decision process, the reason R3 is routing towards
>R1 is the AS-PATH. Distance doesn't come into play until you try to
>install the route in the routing table. BGP best-path selection
>determines which path is best in the BGP table, not which appears in the
>IP routing table.
>
> Based on this, it should be fairly obvious which attribute you
>need to modify (I hope). If not, review Halabi's book or Doyle vol 2 to
>see how these attributes function.
>
> For more info on BGP bestpath:
>
>http://www.cisco.com/warp/public/459/25.shtml
>
>
>HTH
>
>Brian McGahan, CCIE #8593
>Director of Design and Implementation
>brian@cyscoexpert.com
>
>CyscoExpert Corporation
>Internetwork Consulting & Training
>http://www.cyscoexpert.com
>Voice: 847.674.3392
>Fax: 847.674.2625
>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
>Of
> > Nguyen, Thai
> > Sent: Tuesday, August 20, 2002 6:40 PM
> > To: 'ccielab@groupstudy.com'
> > Subject: BGP Challenge
> >
> > Hi all,
> >
> > I've been working on this little problem for few days but have not
>been
> > able
> > to crack it.
> > Can anyone shed light into this please?
> >
> >
> >
> > R1============R2
> > = =
> > = =
> > = =
> > = =
> > ========R3
> >
> >
> > R1 is in AS 1
> > R2 and R3 is in AS2
> >
> >
> > R1 is advertsing a subnet eg 1.1.1.0/24. Normally R3 will have to path
>to
> > the network, first via R1 directly and second via R2.
> > And the preferred path is via R1 because the administrative distance
>of
> > the
> > route learning from R1 is 20 while the route learned via R2 is 200.
> >
> > The challenge of the excercise is to configure R2 so that R3 would
> > prefered
> > the route it learned from R2 without additional configuration on R3.
> >
> > regards
> >
> >
> > Regards,
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
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Hongbing Qin
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