BGP Beginner

Unanswered Question
Dec 16th, 2008
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If I have two internet circuits from two different ISP with public IPs, and if I want to implement load-sharing with BGP on these two circuits. Do I have to get AS # from ARIN? or from the ISP?


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Giuseppe Larosa Wed, 12/17/2008 - 01:52
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Hello James,

only the regional internet registry ARIN can give you a public AS number.

ISPs themselves got their AS numbers from ARIN, RIPE and the other ones


Current RIR policies require real multihoming to get a public AS number because they are becoming rare (actually starting from next year the 32bit As numbers will be introduced)



Hope to help

Giuseppe


james1212 Wed, 12/17/2008 - 08:38
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Thanks for the reply


I have public IPs that I got from two different ISP. Once I have AS#, should I ask my two ISPs to advertise my public IP to internet cloud which will be associated with AS#?


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Giuseppe Larosa Wed, 12/17/2008 - 11:16
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Hello James,

the answer is yes: you need your own AS number if you are multihomed and you need to advertise your own IP block.

Now, you are doing NAT using two blocks given to you by your ISPs.

This kind of scenario usually provides a primary/backup use of the two ISPs.

If you want to have a real load balancing both outbound and in inbound (return path) you need your own AS number and your own public IP address block.

I think you need to apply for both, the AS number and the ip address block, with ARIN.


Hope to help

Giuseppe


james1212 Wed, 12/17/2008 - 13:56
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Hello


Thanks for the info

so I guess I have to contact ARIN to get AS#

and also tell them the public IP addresses that I am give from two different ISP must be associated with the AS#.


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Giuseppe Larosa Thu, 12/18/2008 - 04:58
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Hello James,

you need to apply for your AS# and for your own IP address block.

The ip addresses you are using belong to your ISPs and cannot be assigned to you without their consensus.




Hope to help

Giuseppe


Craig Norborg Thu, 12/18/2008 - 06:55
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The real answer is a combination of several of these. You can either get your own AS from ARIN, or get your two ISP's to agree on a private AS. You are probably much better off getting your own AS from ARIN though, gives you more flexibility.


You don't have to get your own IP address block from ARIN however, you can use an address block from one of the providers. However, you MUST tell the provider your getting the address block from that you will be using it with another ISP, otherwise they might summarize it when they advertise it to other providers at their border.


Why is this important? Lets say ISP A gave you the block of IP's and they set up BGP with you not knowing you're going to do the same with ISP B. If ISP A advertises your subnet to other providers as part of a summarized route, but ISP B advertises just your subnet, what will happen is all traffic that doesn't originate on ISP A's network will come via ISP B. Only traffic that originates within ISP A and their direct customers will come via ISP A.


Why would it do this? Because ISP B is handing out a more "specific" route. ie: to the Internet they will see two possible routes to your network, via ISP A its part of a /8 network, but from ISP B they see it as a /24 network. The more specific route is ALWAYS preferred. The only exception for this will be within ISP A since they are summarizing the route at their borders.


So, IMHO definitely get your own AS from ARIN. But its your choice whether you get your own IP's or not, ARIN will encourage you to get them from one of your ISP's unless your over a certain size. Plus it costs money while getting them from your ISP is probably free.


However, make sure both providers know about the other so they don't make assumptions on how they can configure your routes. I also recommend you use what are called "Looking Glasses" to confirm your routes are being advertised how you want. A looking glass is where different ISP's will let you see how your routes look to them. If you see one ISP advertising you as part of a summary and the other not, you know there is a problem.


Hopefully I covered everything for you!! Feel free to ask questions either via this forum or email.


-Craig

Mohamed Sobair Thu, 12/18/2008 - 02:01
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Hi,


If you dont have a registered Public AS, then the alternative is to have both ISPs agree on asingle private AS.(Its manadatory)


To prevent from ISP mistake, you should also implement the appropriate filtering in place.



HTH

Mohamed

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