Is it time to abandon bogon prefix filters?

Chris Adams cmadams at
Mon Aug 18 09:29:52 CDT 2008

Once upon a time, Sam Stickland <sam_mailinglists at> said:
> I think you misunderstand the meaning of the "ip verify unicasr source 
> reachable-via any" command. When a packet arrives the router will drop 
> it if it doesn't have a valid return path for the source. Since the 
> source is a bogon, and routed to Null0, then the inbound packet is dropped.

First, that is only true on Cisco routers (all the world is not a

Second, you are missing the point: you have bogon route for 10/8, but
rouge route for 10.1/16 (or even 10.0/9 and 10.128/9) arrives; it is
more specific and your automatic bogon filter is useless.

Chris Adams <cmadams at>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.

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