> -----Original Message----- > From: Rafael Ganascim [mailto:[email protected]] > Sent: Thursday, May 20, 2010 11:25 AM > To: [email protected] > 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
