Adverising same route over two AS

Unanswered Question
Jun 8th, 2009


I would like to know the danger by advertising your own prefix through two diffrent AS on internet.

Will the prefix be drop by RAD box ?


I have this problem too.
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Giuseppe Larosa Mon, 06/08/2009 - 03:19

Hello Mohammed,

a public ipv4 prefix should be originated by only one AS number that is the legitimate owner of it.

We can say that current Internet is based on this.

As you say if you originate it also from a non legitimate AS number you are doing something wrong (an inconsistent path) that can be filtered by providers that check the validity of advertisements.

If you are meaning insted that you are peering with two different ISPs this is legitimate and actually is a good reason to get a public AS number.

Hope to help


mohammedrafiq Mon, 06/08/2009 - 03:28

I am peering with diffrent ISPs but with two my diffrent AS( both own by me), like I have AS 10 and peer with ISP1 and I have another As 20 and peer with ISP2, then I have prefix and advertise to ISP 1 and again to ISP2.

Giuseppe Larosa Mon, 06/08/2009 - 04:33

Hello Mohammed,

you may want to consider the neighbor local-as command that can be used in a scenario like yours.


you can use this to pretend to be in AS10 when talking to ISP2 hiding the fact that your second router is actually running in AS20.

ISP2 has to update its configuration accordingly (neigh ... remote-as 10 instead of neigh ... remote-as 20)

Hope to help


Giuseppe Larosa Mon, 06/08/2009 - 06:40

Hello Mohammed,

the risk is that someone (potentially everyone) has the right to discard the route not originated in the legitimate AS number.

Hope to help



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