Re: BGP AS-Path Prepend being ignored on the Internet routers

From: Ronnie Angello (ronnie.angello@gmail.com)
Date: Fri Dec 05 2008 - 17:28:22 ARST

Try prepending the AS number more - 5 times maybe. Even though you
have prepended, your "secondary" path may still have the shorter AS
path.

On Fri, Dec 5, 2008 at 1:42 PM, Piyoush Sharma <piyoush@gmail.com> wrote:
> HI folks,
>
> I have noticed a very strange and curious phenomenon. Let me describe the
> topology first.
>
> There are 2 ISPs each terminating on a different router. At this time there
> is no connectivity between the two routers ( I plan to run iBGP between them
> at the time of this going into production). Router 1 and Router 2 are both
> on one public AS (only 2 routers on this AS - lets call is AS 100)
> Router 1 is connected to ISP 1 - AS 200
> Router 2 is connected to ISP2 - AS 300
>
> Router 1 is advertising network 1.1.1.0/24 to ISP1 eBGP peer
> Router 2 is advertising network 1.1.1.0/24 with AS-PATH prepend AS100 AS100
> AS100 (3 times)
>
> Now the minute I advertise the same network on Router 2, I noticed that
> incoming traffic starts coming across Router 2.
> On checking this on a public router that is receiving full internet bgp
> tables from multiple ISPs, I find that within a few minutes of advertising
> the network on Router 2, the BGP table on it no longer shows the path from
> ISP 1, the only path it shows is from ISP2.
>
> Here it gets curious, I can see that the prepend is working, the AS-PATH
> shows something like this... ASXXX ASYYY ASZZZ AS300 AS100 AS100 AS100 AS100
> But I do not see the path from ISP1 which should be the primary path...
>
> The idea is to achieve redundancy through ISP2 incase ISP1 goes down. One
> work around I have thought of is to use conditional advertising, based on
> tracking from IP SLA (would this work???)
> Wondering if anyone else has seen this issue before...
>
>
> Regards
> Piyoush
>
>
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-- 
Ronald Angello
CCIE #17846

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