Communities BCP [was: RE: BGP Path Filtering]

Danny McPherson danny at
Fri May 16 22:53:00 UTC 2003

> Spank me if I'm wrong here, but you don't pass MEDs on transitively
> anyway. 

Correct.  As a matter of fact, even advertising MEDs to IBGP is left as "an
implementation issue", not a protocol requirement.

> And if you strip communities off, you limit your peers/customers/
> providers from being able to use the same control that you are
> asking for.

As everyone has different policies, they're likely stripped or ignored on
ingress (or somewhere across the world) anyways.  Given, you'd accept them
on ingress from transit peers for RFC1998ish stuff, and perhaps under
special circumstances from bilateral peers, but that doesn't mean you ever
need to carry them past the ingress BGP router into your own network.

The effects of propagating what's essentially garbage needlessly have real
impacts on the convergence and stability characteristics of the global
routing system.  It essentially falls back Postel's robustness principle
with "[...]  be conservative in what you send".

Though I've never put anything formal together (for general consumption), I
have performed testing and seen the effects the presence of needless
attributes have on efficiencies of BGP update packing (and all things
subsequent), and they are very real.


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