BGP Transit AS

Rafael Ganascim rganascim at
Thu May 20 18:25:04 UTC 2010

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?



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