Re: BGP Local Preference

Hello there!

Looks like you have the right idea here for the most part. You are right --
Local Preference influences decisions in your own AS with the highest LP
winning the battle. AS Prepending is a good way to influence how traffic
comes BACK to you. However, highest LP beats shortest AS-Path so if the
remote-AS has a higher LP for a particular route it will select that path
first.

The one thing I did want to clear up is that you can send local-preference
to an eBGP neighbor if you want to.

On Wed, Sep 2, 2009 at 10:37 AM, groupstudy <groupstudy_at_gmail.com> wrote:

> Hi,
>
> I have a question on local preference I'm sure someone in this group can
> clear up for me.
>
> My understanding of Local Preference and how I've always used it is that is
> used to influence path selection within a particular AS
>
> I have a situation where I am trying to influence the way traffic comes
> back
> to my network that I am advertising out to two different ASs. At the same
> time I am using Local Preference to influence path selection within my own
> AS (the higher the local preference the more preferred). For path 1 (ASN
> 2)
> I am advertising my network without any manipulation and for routes learned
> via Path 1, I have set the local preference to 200. For Path 2 (ASN 3), I
> am advertising my network with my local AS (ASN 1) prepended 3 times and
> for
> routes learned from Path 2 I have set the local preference to 150. This
> results in traffic leaving and arriving via Path 1.
>
> Again, This is the way I have always understood it and done it and I
> believe
> this to be correct.
>
> Just for completeness, ASN 2 and ASN 3 have a peering to another AS, ASN 5.
> A router within ASN 3 learns of my network from AS 5 as well as from ASN 1.
> If a site attached to this router needs to get to my location, would it
> select the path through AS5 becasue the path is shorter (AS5 learns my
> network from AS 2 which i am advertising to it without any manipulation),
> or
> would it select the path through its own AS3 if the Local Preference is
> higher within its own AS3?
>
> I am being told AS prepending may not always work because the local
> preference within ASN 3 would override the path length if it was higher.
>
> My understanding is that Local Preference only matters within one AS (not
> exported).
>
> Any help in clearing this up would be greatly appreciated.
>
> Thanks!
> S
>
>
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-- 
Regards,
Joe Astorino - CCIE #24347 R&S
Technical Instructor - IPexpert, Inc.
Cell: +1.586.212.6107
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