Re: bgp best path confusion

I believe that it is step 12 for your initial route and IGP metric for the
second one, based on this doc
http://www.cisco.com/en/US/customer/tech/tk365/technologies_tech_note09186a0080094431.shtml.
If you read carefully cisco added this little note on step 11 (Router ID
comparison for iBGP) which states "*Note: *If a path contains route
reflector (RR) attributes, the originator ID is substituted for the router
ID in the path selection process.", therefore you have the same router id
2.2.2.2 and 2.2.2.2 (originator-id) for this path, so you have to move on
to the next step. Next step says shortest cluster-list, which in this case
is the one without a list (0 vs 1) and that's why the second path is
selected as your best path.

I would recommend going thought the cisco doc (see link above), using gns3
to create a topology where you can test each decision point to really
understand how cisco BGP best path decision process works, oh and make sure
to know how to navigate to it during the lab.

Hope that helps.

Tom Kacprzynski
CCIE#36159 (R&S)

On Wed, Aug 29, 2012 at 10:29 PM, HEMANTH RAJ <hemanthrj_at_gmail.com> wrote:

> Hi
>
> It must be the IGP METRIC to reach the NEXT-HOP in BGP or else CLUSTER
> LENGTH which will decide this when you have redundant RR. Check out
> whether there both the RRs are clients to each other or non clinets . If
> they are clients to each other, then u must have a unique RR Cluster ID in
> each of them.
>
>
> On Thu, Aug 30, 2012 at 7:14 AM, Joe Sanchez <marco207p_at_gmail.com> wrote:
>
> > I would save router-id.
> >
> > Regards,
> > Joe Sanchez
> >
> > ( please excuse the brevity of this email as it was sent via a mobile
> > device. Please excuse misspelled words or sentence structure.)
> >
> > On Aug 29, 2012, at 11:57 AM, oo IPX <oispxl_at_gmail.com> wrote:
> >
> > > i have a hub and spoke topology, R1 being hub.
> > >
> > > R1-is a Route reflector.
> > >
> > > also, R1, R2 and R3 are fully meshed ibgp peers. with R1, R2 and R3
> > > advertising networks 1.0.0.0/8, 2.0.0.0/8 and 3.0.0.0/8
> > >
> > > *on what level of the bgp path decision making the following route is
> > been
> > > chosen?*
> > >
> > >
> > >
> > > *R3(config-router)#do sh ip bgp 2.0.0.0
> > > BGP routing table entry for 2.0.0.0/8, version 4
> > > Paths: (2 available, best #2, table Default-IP-Routing-Table)
> > > Flag: 0x820
> > > Not advertised to any peer
> > > Local
> > > 10.1.12.2 (metric 1) from 10.1.13.1 (1.1.1.1)
> > > Origin IGP, metric 0, localpref 100, valid, internal
> > > Originator: 2.2.2.2, Cluster list: 1.1.1.1
> > > Local
> > > 10.1.12.2 (metric 1) from 10.1.12.2 (2.2.2.2)
> > > Origin IGP, metric 0, localpref 100, valid, internal, best
> > > *
> > >
> > >
> > > Network Next Hop Metric LocPrf Weight Path
> > > *>i1.0.0.0 10.1.13.1 0 100 0 i
> > > ** i2.0.0.0 10.1.12.2 0 100 0 i
> > > *>i 10.1.12.2 0 100 0 i*
> > > *> 3.0.0.0 0.0.0.0 0 32768 i
> > >
> > >
> > >
> > > Regards
> > >
> > >
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>
> --
> Problems arise because we talk,problems are not solved because we don't
> talk So good or bad talk to your affectionate one's freely.
>
> Yours Friendly,
> H P HEMANTH RAJ
> CCIE#28593 (R&S)
>
>
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Received on Thu Aug 30 2012 - 10:44:38 ART