From: PANDI MOORTHY (moorthypandi@gmail.com)
Date: Fri Mar 07 2008 - 16:28:22 ARST
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.html#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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