Re: BGP Transit AS

[joel jaeggli <joelja () bogus com>]

On 2010-05-20 11:25, Rafael Ganascim wrote:
> 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...

longest match wins...

if you're customer 1

deaggregate the avertisement to isp 1 or re-aggregate the advertisement 
to nsp 1. either will achieve the same end.

if you're isp1 consider what customer 1 was trying to achieve by doing 
this. e.g. they're traffic engineering (or they are clueless) and 
theirfore have a vested interest in the current path.

> Is this solution right ? What is the better solution for this
> scenario? How large ISPs solve this kind of problem?
>
>
> Thanks,
>
> Rafael