From: Piyoush Sharma (piyoush@gmail.com)
Date: Fri Dec 05 2008 - 17:19:38 ARST
Luca,
Advertised on router 1 - very long time ago... xx months - No advertiment on
router 2 at this time.
Pre-pending on router 2 when advertising the network - waiting atleast 8
hours
Then checking on the same route server... it shows only the route from
Router2 with the prepend
I hope this answers your question.
On Fri, Dec 5, 2008 at 11:01 AM, Luca Hall <lhall@setnine.com> wrote:
>
>
> advertising them both, waiting a good amount of time
> then on a public route server you see just the router 1
> advertised route. then you do pre-pending on router 2 only,
> wait another good amount of time then on the same route server
> only see the router 2 advertised route? is that what your saying?
>
>
>
> ----- Original Message -----
> From: Piyoush Sharma <piyoush@gmail.com>
> To: Cisco certification <ccielab@groupstudy.com>
> Sent: Fri, 5 Dec 2008 13:42:43 -0500 (EST)
> Subject: BGP AS-Path Prepend being ignored on the Internet routers
>
> 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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