External BGP route in an AS

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Nov 28th, 2006
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Currently have BGP set up at our NOC through two upstream peers, for 4 Class C networks. We want to split off a /26 to a remote site, using a third, different ISP for client use. What's the best way to advertise the route to this new subnet? Just advertise the new route through our current BGP speaker, or? Thanks.

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tdrais Tue, 11/28/2006 - 07:30
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Your biggest issue is that most ISP's will not accept /26. Even if they do their upstream BGP neighbors will more than likely filter to /24.

If this was for example a /22 and a /24 all you would do is have your new third isp advertise the network and continue to advertise the summaries into your other 2 isp's since the more specific route will be preferred all will work.

This same theory works with the /26 but it will be a matter if all the ISP's involved allow it. It might if the third ISP is a customer of both your current ISP and they configure things to make it work.

royalblues Tue, 11/28/2006 - 07:49
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I agree with Tim.

I had a similar requirement wherein i had to advertise /27 subnets, but these were rejected by my ISP.

The least which can be advertised is a /24 subnet.



jreusch Tue, 11/28/2006 - 07:59
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Thanks to both, I thought the same issue might arise... Would you think a PIX-PIX VPN tunnel between here and there, thus putting both routers in the same space would be a viable option? At our end is 45Mbs Fiber, there is 10 Mbs Copper-Fiber. Thanks again.


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