Advertising BGP-4 from two islands

Fouant, Stefan Stefan.Fouant at
Sun Sep 13 02:01:49 UTC 2009

This is considered a normal and accepted practice, and there are many companies out there that do just this sort of thing.  From the perpective of everyone else outside your AS everything will be perfectly fine.  The only thing you'll need to be aware of is that your islands will not be able to see each others routes due to AS Path loop prevention mechanisms inherent in BGP, since they will see routes received with your AS in the AS Path and will assume some type of loop is taking place.  You can get around this by configuring AS loop support on your routers.

My apologies for the top-posting, I'm using my BB.

Stefan Fouant 
Neustar, Inc. / Principal Engineer
46000 Center Oak Plaza Sterling, VA 20166
Office: +1.571.434.5656 ▫ Mobile: + ▫ GPG ID: 0xB5E3803D ▫ stefan.fouant at

----- Original Message -----
From: Francois Menard <francois at>
To: nanog at <nanog at>
Sent: Sat Sep 12 21:35:00 2009
Subject: Advertising BGP-4 from two islands

I have an opportunity to launch services in a remote marke, where I  
cannot extend my backbone to.

However, this market is big enough that I can afford to put a Cisco  
7201 over there and peer in BGP-4.

Do you have any advice as to what may happen if I advertise different  
blocks from the same AS number, from two different locations, one of  
which I do not have my own transport facilities to...


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