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Two companies wants to exchange some private prefixes (4K prefixes) using eBGP. Each company will have two border routers (with OSPF as IGP, and mutual redistribution & aggregate in eBGP)

So, this is a dual-homed design between the two companies.

Do you advise to run iBGP between the two border routers inside each company, or stick with only an IGP between them? Is there any best practice in this case?

AFAIK, iBGP would be good if I'm redistributing a huge amount of prefixes (INET table), but I couldn't see any other real benefits for this case.

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Disclaimer: This all really depends on information you haven't provided, and there could be circumstances which dictate you do it differently, e.g. you only want to advertise some internal routes through one border router, and some through another.

If you run iBGP between the two border routers, you avoid redistribution problems, which could cause routing loops. The two border routers would be able to directly exchange BGP prefixes without the fear of creating routing loops because of the iBGP rule about not advertising prefixes which were learned through iBGP to another iBGP speaker.

If you only run an IGP between them, you must redistribute BGP prefixes into the IGP, and vice versa, in both directions, and, without careful planning and configuration, you could re-advertise the same prefixes back to the source.

It's really just easier and safer to use iBGP between the border routers to advertise prefixes learned through eBGP. If you have more than two border routers, you need to remember that iBGP requires a full mesh between the iBGP speakers.

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  • But if I do this, all my internal routers need to run BGP as well, which sometimes is difficult regarding the licenses, capacity... In my case ony the two borders can do BGP, not the others which run only OSPF, so I will need to do reditribution in all cases. Do I still need to run iBGP between borders ? – ClearShell Apr 17 '16 at 7:20
  • No, they don't. You either need to redistribute the external routes into your IGP, or you may be able to use a default route in the IGP. The key is that you don't need to redistribute back into BGP, fixing the mutual redistribution problem, since you can use network statements in BGP to advertise your local networks to the other AS. The border routers learn the external routes the other has through iBGP, so you don't need to redistribute IGP to BGP so that each border router knows what the other has. You should only redistribute if you really need to, not for convenience. – Ron Maupin Apr 17 '16 at 13:33

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