Re: BGP Path Selection - eBGP vs iBGP

Really simple actually, the RIB selection process, does not apply at all.
BGP chooses a best route and enters that into the RIB.

As aundra siad the tie breaker here is point 7 in the BGP selection
process, (eBGP before iBGP) (girlfriend before wife)

:)

On Tue, Feb 16, 2010 at 5:20 PM, Ruhann <groupstudy_at_ru.co.za> wrote:
> 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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>>
>

-- 
<ruhann>
www.routing-bits.com
Blogs and organic groups at http://www.ccie.net