BGP Transit AS

Michael K. Smith - Adhost mksmith at adhost.com
Thu May 20 14:49:54 CDT 2010


> -----Original Message-----
> From: Rafael Ganascim [mailto:rganascim at gmail.com]
> Sent: Thursday, May 20, 2010 11:25 AM
> To: nanog at nanog.org
> Subject: BGP Transit AS
> 
> Hi all,
> 
> I have a doubt about the bellow scenario, where the ISP1 use eBGP
> sessions to its peers and is a BGP Transit AS.
> 
> 
>   NSP 1 ------------------ ISP 1 Router2 ----------- NSP 2
>    |                             |
>    |                             |
>    |                             |
>    | annunce /21          |
>    |                             |
>   Customer1 --------------- ISP 1 Router1
>           announce /20
> 
> 
> The "Customer1" is client on both ISPs (ISP1 and NSP1) and have an /20
> IP prefix. To NSP1, it announce two /21 prefixes. To ISP1, it announce
> a /20 prefix. If traffic comes from NSP 2 (connected only to ISP 1) to
> Customer1, the ISP 1 Routers try to send data over NSP 1, ignoring the
> Custormer1->ISP1 link.
> To solve this question, an solution that I found is filter Customer1
> prefixes in BGP session between NSP1 and ISP1 Router2. But this don't
> appear scalable...
> 
> Is this solution right ? What is the better solution for this
> scenario? How large ISPs solve this kind of problem?
> 
The more specific /21's are winning over the /20, so they will always be preferred by default.  If you want to change that, you could announce the /20 to NSP1, or announce the /21's to ISP1.

Mike




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