BGP Local Preference

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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Received on Wed Sep 02 2009 - 10:37:36 ART