Blackhole Routes

Richard A Steenbergen ras at
Sun Oct 3 02:54:56 UTC 2004

On Sat, Oct 02, 2004 at 11:06:31PM +0100, Ian Dickinson wrote:
> You'd need an additional community to flag this eg. 65001:666 means to
> blackhole, 65001:6666 means to propagate it as well.  I can't speak for
> others but when we blackhole the destination (as opposed to blackholing 
> the source or mitigating) we often only do it in the direction from
> which the attack is coming*.  Why drop globally when you can drop
> traffic from a subset of the Internet?  Your victim will thank you
> if 90% of their customer base can reach them, versus none.  Similarly,
> if they're multi-homed, they may well rely on you NOT propagating.
> Maybe this looks different from the perspective of a global Tier-1.

No, 65001:666 (or whatever value is chosen for a well known community, for 
the sake of argument) means to set the next-hop to something that discards 
packets, and otherwise propagate the route as normal. If you don't want it 
to be exported in a specific direction, you add no-export or no-advertise 
or just don't advertise it to peer X just like you would do with any other 
route. Don't complicate the protocol unnecessarily based on your specific 
assumptions of how you might or might not use a feature.

There is nothing more or less complicated about this than adding a value 
to the end of 
and declaring it a standard blackhole community. How you use it, how you 
export it, and who you accept it from, are provider specific policy 
decisions. However, based on the knowledge that a blackhole community 
route is no different than a regular route in its ability to cause 
unreachability if incorrectly announced, I would tend to suspect that most 
people would choose to allow this to be propagated globally.

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

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