BGP Redundendcy

Unanswered Question
Apr 17th, 2007


If i advertise 2 prefixes,let say and to two diffrent ISP-A and ISP-B trough two difrrent AS,100 aggined by ISP-A to us and AS200 owned by us.ISP-B is primary.Will my traffic fallover to ISP-A in case of link failure to ISP-B?.


I have this problem too.
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Danilo Dy Tue, 04/17/2007 - 04:43


Pardon me to clarify what you say, you mean;

1. You have two prefixes and

2. You are connected to two ISP, ISP-A and ISP-B

3. You have two ASN; ISP-A owns ASN100 and assigned it to you. You own ASN200.

4. Now you want to BGP peer with ISP-A and ISP-B advertising two prefixes and two ASNs

5. You want ISP-B as primary and ISP-A as backup.


1. Do you own the two prefixes? If not, please clarify who owns it and assign to you.

2. How many routers you have? Do you only have one router connected to both ISP-A and ISP-B?

mohammedrafiq Tue, 04/17/2007 - 05:17

Hi Medan,

yes thats I mean,


1- I own and few more, but is own by ISP-A and can only be advertised through them, thats no problem,

2-two routers, 1 for ISP-A connection and 2nd for ISP-B connection.


Danilo Dy Tue, 04/17/2007 - 05:23


You can advertise your own prefixes and ASN to both ISP, but you cannot advertise ISP-A prefix and ASN to ISP-B.

Since you have your own prefixes and ASN, to make things simple, why don't return the prefix and ASN to ISP-A and migrate the hosts using ISP-A IP Address to your own IP Address? Using ISP-A prefix and ASN, you will always be tied with them.

mohammedrafiq Tue, 04/17/2007 - 05:51


Bit correction, I will have 1 bgp peering trough AS100 to ISP-A advertising and, and 2nd through AS 200 to ISP-B adverting, I only need redundency to and mu worry is 1.1.1./0/24 is adverise to two isp's through 2 diffrent AS, will it failover?

Yes its ok to get rid of ISP-A own prefis, but i has been used and will take us long time to migrate,

royalblues Tue, 04/17/2007 - 06:20


Ideally when you advertise your own prefixes to the internet, it has to be updated in the internet routing database as well eg. RadB which most ISP look into before accepting the route.

While updating the RadB it is mandatory to tell from which AS the particular route will be advertised. So some providers might reject if they see the route via 2 ASN.

A better option would be to run a single BGP session with your own AS between both the ISPs.

HTH, rate if it does



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