Quick question about inbound route-selection

Wayne E. Bouchard web at typo.org
Thu Jul 16 23:05:04 UTC 2009

On Thu, Jul 16, 2009 at 06:32:32PM -0400, Deepak Jain wrote:
> > As for trying to determine where your inbound traffic is coming from by
> > looking at natural bgp, this is absolutely impossible to do correctly.
> > First off, your inbound is someone else's outbound, and the person
> > sending the traffic outbound is in complete and total control. The vast
> > majority of the traffic on the Internet is being picked by local-prefs
> > based on policies like "what does this make/cost me monetarily" or
> > "which major networks can I grab in a simple as-path regexp to balance
> > some traffic". But even if you ignore all of that, the "natural" path
> > selection is based on criteria which is specific to the other network
> > or
> > even to a specific session which you can't possibly know about remotely
> > (e.g. their router id).

I would actually disagree with that and go one step further. Look at
content providers. They're not concerned about best path. They're not
even concerned about shortest path. Since bandwidth consuming services
are what they provide, they're interested in cheapest path as much as
they are the shortest path.

> Another way to say what Richard is getting at (which was full of good information) is:
> Just because you aren't modifying what your BGP process sees, at this stage of the Internet's maturity, it is safe to assume almost everyone else is. Therefore, rather than pray for BGP to make a logical selection, even though its *probably* being fed prefs based on other people's engineering, you should take charge of the parts you can.

 Take the traffic shaping products. They completely override the
normal BGP mechanisms and force traffic out a given circuit. So as
long as there is a usable route down that interface, it will get used
whether the neighbor wants it or not.

The long and short of it is that via MEDS, prepending, and your
neighbor's community policies, you can *hint* where you want traffic
to come in but ultimately you may have very little say in the matter.
(Community exchanges are probably the best mechanism since the
existance of them in your peer's network means they will be most
likely to honor your hints.)

As Deepak indicated, don't rely on the originally the protocol's best
effort. Take control of your own world wherever you can. It's the only
way to ensure a good measure of predictability.


Wayne Bouchard
web at typo.org
Network Dude

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