From: Sharifi, Reza (Reza.Sharifi@gdit.com)
Date: Fri Mar 07 2008 - 19:39:05 ARST
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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