Re: BGP Local Preference

Thanks Joe. I thought Local Preference was not exportable.

I suppose you would just configure the same type of route setting the local
preference and apply outbound to the peer?

Thanks

On Wed, Sep 2, 2009 at 11:23 AM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> 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
> Fax: +1.810.454.0130
> Mailto: jastorino_at_ipexpert.com

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Received on Wed Sep 02 2009 - 11:54:40 ART