Re: BGP Path Selection - eBGP vs iBGP

Thanks Andre for the fantastic input....
Clears all doubts,
Thanks

On Tue, Feb 16, 2010 at 3:07 PM, Dufour, Andre <Andre.Dufour_at_paetec.com>wrote:

> Hello,
>
> The answer is not because eBGP is 20 and iBGP is 200. These AD numbers are
> sorted out AFTER the BGP best path selection process is done.
>
> Let's look at this case.....
>
> R5 will see both paths....
> 1 - weight - tie
> 2 - local pref - tie (the default local pref is 100 unless you set it
> otherwise)
> 3 - locally originated - tie; this network is not locally originated (i.e
> aggregate, network, or redistribute command was not done on R5)
> 4 - AS path length - tie - both have 1 AS in the path
> 5 - lowest origin type - tie - (only one router is advertizing the path and
> it was via the network command)
> 6 - lowest MED - tie
> 7 - -----> prefer eBGP over iBGP (and NOT because of AD - AD is only
> sorted out after the best path process has been done).
>
> Let me know if this answers your question.
>
> Andre
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Garth Bryden
> Sent: Tuesday, February 16, 2010 3:27 AM
> To: Cisco certification
> Subject: BGP Path Selection - eBGP vs iBGP
>
> Hello Group!
>
> I have what is probably a fairly simple question, but I am slightly
> confused with my BGP Labs.
>
> I have a simple topology.
>
> R1 and R3 are in AS 1 and iBGP Peers
>
> R4 and R5 are in AS 2 and iBGP Peers
>
> R1 and R4 are eBGP Peers
>
> R3 and R5 are eBGP Peers
>
> I have the network 10.0.0.0/16 being advertised by R3
>
> On R5 I recieve the NLRI with two paths. One is eBGP via R3 the other iBGP
> via R4... R4 sends the iBGP prefix with a Local Preference of 100 and of
> course since it's external R3 doesn't send a Local Preference attribute...
> Now since the second method of best path selection is "Highest Local
> Preference" why do my routers choose eBGP over iBGP?
>
> Here is the BGP RIB.....
>
> Network Next Hop Metric LocPrf Weight Path
> * i10.0.0.0/16 155.1.45.4 0 100 0 2 i
> *> 155.1.0.3 0 2 i
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Tue Feb 16 2010 - 16:15:36 ART