RE: BGP Path Selection - eBGP vs iBGP

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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Received on Tue Feb 16 2010 - 09:07:04 ART