-----Original Message----- From: Rafael Ganascim [mailto:rganascim@gmail.com] Sent: Thursday, May 20, 2010 11:25 AM To: nanog@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