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New Member

BGP advertisment

Is it:


B)bad design

C)absolutely forbidden

to advertise the same network originating out of two different ASs?

Not to the Internet but other EBGP peers. The two ASs are exchanging routes via an IGP.

Cisco Employee

Re: BGP advertisment

Not necessarily permissable, bad design nor absolutely forbidden on a private network design ;o) It largely depends on what you need to achieve. IOS will not prevent you from doing it.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600

Re: BGP advertisment

If you have to choose one of the choices I would say 'permissible' is probably the closest one. Infact, I have seen many customers multi-homed to two or more ISPs having their only IP block, belonging to one SP, advertised by other SPs out to the Internet for redundancy purposes. Ofcourse, the other ISPs would have to agree to route traffic for this network on the Internet.



New Member

Re: BGP advertisment

Thank you both for your quick responses. This was my 1st post here so I didn't know what to expect, hence my rather limited description. Let me expand. This is in an extranet environment running mostly private ASNs, no ISP connectivity. I have two data centers, each running under its own ASN. To accomplish failover I am advertising some of the same networks out of both data centers to the extranet peers. It is built and functioning OK. But one of the business partners expressed concern that I was advertising the same network to him with different originating ASNs. My concern would be that this could cause problems for the BGP AS path algorythm. It sounds like neither of you expect that it would. Then my next question is, is this a legitimate BGP config? Probably not a best pratice? Or perhaps OK to do as explained by Sundar. Thank you. Kevin


Re: BGP advertisment


As others described, it is permissible, routing options available..... See but it will looks inconsisistent network from the other ASNs prespective who is receiving the same route from different AS No.s.... It won't pose any probs as far as the alogorithm is concern....

But as you asked, you can say it is not Best routing Practice......

You can see this kind of configuration on field and you can very well ask you parter to choose desired one of them using weight or local pereference attribute to override AS-Path.

Hope it clears!


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