From: Hash Aminu (hashng@gmail.com)
Date: Sat Mar 08 2008 - 08:44:34 ARST
Gary you are right...BGP will prefer an EBGP over IBGP route due to
the AD ...no complications here...
The philosophy is that ....I will prefer to use IGP instead of IBGP to
route within my AS (thats why IBGP AD is higher than any IGP out
there) but will prefer EBGP over IBGP or IGP to get to networks
outside my AS(thats why EBGP AD is lower than IGP or IBGP).
HTH
Hash
On 3/8/08, Gary Duncanson <gary.duncanson@googlemail.com> wrote:
> Reza,
>
> I also think the ebgp route wins here. Ebgp has a lower AD than ibgp. As
> for Local Pref I think the router would prefer the higher value as it is in
> the AS but will not get that far in the route selection process.
>
> Regards
> Gary
>
> ----- Original Message -----
> From: "Sharifi, Reza" <Reza.Sharifi@gdit.com>
> To: "PANDI MOORTHY" <moorthypandi@gmail.com>; <subodh.rawat@wipro.com>
> Cc: <funccie@gmail.com>; <ccielab@groupstudy.com>
> Sent: Friday, March 07, 2008 9:39 PM
> Subject: RE: Confusion on BGP route selection
>
>
> > Pandi,
> >
> > As you can tell by the name, local-preference is only "local" to your
> > AS. Other AS numbers will never see your local-preference. Also, the
> > reason that EBGP is preferred over IBGP is because EBGP offers better
> > loop detection and prevention than what is available in IBGP.
> >
> > HTH
> > Reza
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> > PANDI MOORTHY
> > Sent: Friday, March 07, 2008 1:28 PM
> > To: subodh.rawat@wipro.com
> > Cc: funccie@gmail.com; ccielab@groupstudy.com
> > Subject: Re: Confusion on BGP route selection
> >
> > Hi
> >
> > My understanding is BGP path selection works as in the URL
> >
> >
> >
> > http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfbgp.h
> > tml#wp1000898
> >
> >
> >
> > If you set the local preference at R1 for the routes learned from its
> > ebgp
> > peer to 50, then R1 will choose the IBGP route as the best path because
> > of
> > the highest local preference (100)
> >
> >
> >
> >
> >
> > Router will choose the (eBGP) path over the iBGP path only when the
> > steps
> > 2-8 are skipped
> >
> >
> >
> > *1. *If the next hop is inaccessible, do not consider it.
> >
> > This decision is why it is important to have an IGP route to the next
> > hop.
> >
> > *2. *If the path is internal, synchronization is enabled, and the route
> > is
> > not in the IGP, do not consider the route.
> >
> > *3. *Prefer the path with the largest weight (weight is a Cisco
> > proprietary
> > parameter).
> >
> > *4. *If the routes have the same weight, prefer the route with the
> > largest
> > local preference.
> >
> > *5. *If the routes have the same local preference, prefer the route that
> > was
> > originated by the local router.
> >
> > For example, a route might be originated by the local router using the
> > *network
> > bgp* router configuration command, or through redistribution from an
> > IGP.
> >
> > *6. *If the local preference is the same, or if no route was originated
> > by
> > the local router, prefer the route with the shortest autonomous system
> > path.
> >
> >
> > *7. *If the autonomous system path length is the same, prefer the route
> > with
> > the lowest origin code (IGP < EGP < INCOMPLETE).
> >
> > *8. *If the origin codes are the same, prefer the route with the lowest
> > MED
> > metric attribute.
> >
> > This comparison is only made if the neighboring autonomous system is the
> > same for all routes considered, unless the *bgp always-compare-med*
> > router
> > configuration command is enabled.
> > ------------------------------
> >
> > *9. *Prefer the external BGP (eBGP) path over the iBGP path.
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > I assume that R1 has route to both next-hops
> >
> >
> >
> >
> >
> >
> >
> > Regards
> >
> > Pandi
> >
> >
> > On 3/4/08, subodh.rawat@wipro.com <subodh.rawat@wipro.com> wrote:
> >>
> >> Dave,
> >>
> >> eBGP will be prferred over iBGP.
> >>
> >> Thanks
> >> subodh
> >>
> >> -----Original Message-----
> >> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> > Of
> >> dave dave
> >> Sent: Tuesday, March 04, 2008 2:09 PM
> >> To: Cisco certification
> >> Subject: Confusion on BGP route selection
> >>
> >> Hi Expert,
> >> As in below example i am running only BGP, the route 155.1.5.0/24
> >> learning from AS-1 EBGP peer as well as IBGP peer, but my router
> >> selecting EBGP learn route as best route over the IBGP.
> >>
> >> As per my understandig IBGP shpuld be BEST route for 155.1.5.0/24
> >> because of highest LOF of IBGP is 100 (heigest prefer) preferable over
> >> EBGP.
> >>
> >> #R1 (belong to AS 2)
> >> Network Next-Hop Metric LocPrf Weight
> >> Path
> >> * i155.1.5.0/24 155.1.13.3 0 100 0
> >> 1 i
> >> *> 155.1.146.4
> >> 0 1 i
> >>
> >> Please clear my confusion on above.
> >>
> >> Regards
> >> Dave
> >>
> >>
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