[126537] in North American Network Operators' Group
RE: BGP Transit AS
daemon@ATHENA.MIT.EDU (Michael K. Smith - Adhost)
Thu May 20 15:50:10 2010
Date: Thu, 20 May 2010 12:49:54 -0700
In-Reply-To: <AANLkTikgqsXwFd-7ta7fqnXRwr01E0l8yPrUbKvZs-6b@mail.gmail.com>
From: "Michael K. Smith - Adhost" <mksmith@adhost.com>
To: "Rafael Ganascim" <rganascim@gmail.com>,
<nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
> -----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
>=20
> Hi all,
>=20
> I have a doubt about the bellow scenario, where the ISP1 use eBGP
> sessions to its peers and is a BGP Transit AS.
>=20
>=20
> =A0NSP 1 ------------------ ISP 1 Router2 ----------- NSP 2
> =A0 =
|=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0 |
> =A0 =
|=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0 |
> =A0 =
|=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0 |
> =A0 | annunce /21=A0=A0=A0=A0=A0=A0=A0=A0=A0 |
> =A0 =
|=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0 |
> =A0Customer1 --------------- ISP 1 Router1
> =A0=A0=A0=A0=A0=A0=A0=A0=A0 announce /20
>=20
>=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...
>=20
> Is this solution right ? What is the better solution for this
> scenario? How large ISPs solve this kind of problem?
>=20
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
