From: Piyoush Sharma (piyoush@gmail.com)
Date: Fri Dec 05 2008 - 16:42:43 ARST
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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