Re: BGP Local Preference

Yes, you could do that. Now weather the other end cares about it is another
story :P Some ISPs will allow you to set local preference on things to
influence routing, some will not.

On Wed, Sep 2, 2009 at 11:54 AM, groupstudy <groupstudy_at_gmail.com> wrote:

> 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
>>
>
>

-- 
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
Blogs and organic groups at http://www.ccie.net