Re: BGP Path Selection - eBGP vs iBGP

Agreed, the decision is not made on AD.
But AD however is always evaluted before the BGP selection process, or
any other protocols internal decision process. (Here there is no tie
in AD from multiple routes in the RIB, because only ONE candidate
route announced. So this rule never kicked in)

Excluding multipath, BGP like any other protocol, selects a single
best route to be candidate for insertion into the RIB. The decision in
this scenario is made within the BGP process, by using the BGP
attributes. As established BGP decides on the best route being eBGP
over iBGP because all other attribute rules match.

In this case, only one BGP route is candidate to be entered in the
RIB. The route entered happens to be a eBGP route. No other candidate
routes match the prefix length, so the eBGP route wins the spot in the
RIB.

I did a proccess flow about the IGP selection process that also
references a BGP flow process that Richard Bannister did.
http://blog.ru.co.za/2010/01/07/rib-route-selection/

-- 
<ruhann>
www.routing-bits.com
On Tue, Feb 16, 2010 at 4:07 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
>
>
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