From: Gary Duncanson (gary.duncanson@googlemail.com)
Date: Sat Mar 08 2008 - 07:38:37 ARST
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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