Communities

Tue Oct 16 08:25:45 UTC 2001

On Mon, 15 Oct 2001, Jeff Aitken wrote:

> > How many networks are there that use communities to indicate where (which
> > interconnect point) a route was learned?

> How feasible is it for me to provide this information in any
> meaningful way if I have tens or even hundreds of interconnect
> points in my network?

Hm, are there hundreds of interconnect points, even world wide?

> Obviously I can assign a unique community
> to each such point on my network, and tag all routes I learn there
> with that community, but is the benefit of my doing so?  Unless
> you have some way of knowing whether interconnect point A is "better"
> than interconnect point B, how would you use that information?

If two networks are both rather large and interconnect in many places, it
may be hard to put this information to use. But for multihoming customers
this shouldn't be a big problem. For instance, we are in Europe and we
assign a lower local preference to routes our upstreams receive in the US.
So if there is a route over an interconnect point in Europe, we prefer it,
regardless of AS path length. Obviously this will not guarantee selection
of the best path, but there are cases when it prevents a transcontinental
detour.

> > And how many networks use this information if their upstream provides it?

> Without having a clear understanding of each upstream's network
> topology and routing policy, how would you use such information to
> label one route as "better" than another?

Give your multihomed customers some credit. They know how the traceroute
program works. If part of you network or an exchange point is congested,
your customers will know. Why not give them the tools to route around the
problem?

> What problem(s) are you trying to solve, and are you sure that
> BGP communities are the right tool for the job?

The problem is that the BGP route selection algorithm is far from perfect.
Setting the local preference based on the AS path and communities is the
only tool (apart from a big bag of money that makes all the problems
disappear).

Iljitsch