[nog] BGP communities

[Vasil Kolev]

On Sat, 22 Oct 2016 19:53:42 +0300
Marian Marinov <mm at 1h.com> wrote:

> 
> Basicaly I want to be abel to choose which upstream (from the ones
> that my ISPs have) to use. This would allow me to reroute
> destinations around certain routing issues in the world.
> 
> 
> 

For inbound traffic, this will work with adding communities that
prepend, and is pretty standard. For outbound traffic, you can pick
what to use, but not for everything, e.g. you can take the routes that
go through a specific upstream, but without some magic (which isn't
directly related to communities) you cannot pick a different upstream
for some destination.

Let's say the ISP has upstream A and B. 1.0.0.0/8 is routed through A,
2.0.0.0/8 is routed through B. The ASPATH for 10.0.0.0/8 is "ISP A",
for 2.0.0.0/8 is "ISP B" and you can just match the second AS in the
ASPATH and that's it. But there is no way with the usual stuff to say
to the ISP to send traffic for 2.0.0.0/8 through ISP A. There are
tricks like having two separate links/peering sessions that go to
different VRFs that have different upstreams, but most of these are
weird, hard to setup and most ISPs don't bother.
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