When to use public AS number internally?

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Feb 3rd, 2010
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Good Evening,

I currently peer with two different service providers via EBGP and i want to extend BGP internally.  Will i use my public AS internally and peer via IBGP or is it better to use a private AS number internally?  Also, if i use a private number internally will i need to strip that from the AS path before it hits my provider or is that something they do?

Thank you in Advance.

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milan.kulik Thu, 02/04/2010 - 00:27
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if you are using your public AS number to peer with your providers, they will not be able to strip the private AS numbers from the AS-PATH (it fails in a case of private/public AS number mixture within the AS-PATH).

So you would have to strip before advertising to the providers. (I suppose we are talking about Internet connections, in a case of an enterprise MPLS network without an Internet peering private AS could remain unstripped if not clashing.)

As long as you are able to live with iBGP limits (full-mesh or synchronization, etc.), I'd recommend keep using your public AS number.



Giuseppe Larosa Thu, 02/04/2010 - 05:11
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Hello Darren,

the use of your own public AS number is common best practice because a router can run only one BGP process.

It is important to be able to discriminate between iBGP sessions and true eBGP sessions.

In a very big scenario like some service providers BGP confederations are used to divide the space iN multiple mini AS, but this is not probably your case.

Hope to help


Darren Sasso Sun, 02/07/2010 - 09:46
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Got it.  Thanks for the input guys.  So basically i'll have to use my public AS internally and

either use route reflectors or conferderations depending on the number of peers.

Thanks Again.


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