HE.net BGP origin attribute rewriting

Thu May 31 11:55:28 UTC 2012

On May 31, 2012, at 7:26 AM, Nick Hilliard <nick at foobar.org> wrote:
>   There are many useful ways to build a
> multi-exit discrimination policy.  Using origin is not one of them, in my
> opinion.
> 
> The problem is that origin is ranked one place higher than MED.  So if you
> don't rewrite it, you are automatically giving your upstreams an inherent
> means of strongly influencing the tie-breaking policy.  If this were an
> attribute which actually meant something, then maybe there would be some
> point in paying attention to it, but it conveys no useful information these
> days.  IOW, it is completely pointless these days and you almost certainly
> want to work the possibility of any upstream tweaking it.
> 
> Nick
> 

I disagree.  Origin is tremendously useful as a multi-AS weighting tool, and isn't the blunt hammer that AS_PATH is.  The place where I've gotten the most benefit is large internal networks, where there may be multiple MPLS clouds along with sites cascaded off of them - it provides a way of sending "soft" preferences down the transitive chain.  Also useful is "set origin egp XX" - on a route injector, that can post-pend an ASN and limit the spread of a route while still allowing the same transitive properties.

David Barak

Sent from a mobile device, please forgive autocorrection.