On Tue, Oct 11, 2022 at 7:03 PM William Herrin <bill@herrin.us> wrote:
On Tue, Oct 11, 2022 at 5:32 PM Matthew Petach <mpetach@netflight.com> wrote:
[...]
All TCP/IP routing is more-specific route first. That is the expected
behavior. I honestly don't fathom your view that BGP is or should be
different from that norm. If the origin of a covering route has no
problem sinking the traffic when the more-specific is offline, I don't
see the problem. You shouldn't be taking them offline with route
filtering.

*facepalm*

Right.  That's the entire point I started off the subthread with.

The problem lay with an organization that *did* have a problem
sinking the traffic when the more-specific was not available.
They had chunked up their allocation into smaller pieces 
which were distributed to different island locations with no 
internal network connectivity to the island sites.

They were announcing a covering prefix for all the more 
specifics, where the covering less specific announcement 
had no reachability to the more specifics; so when a network 
filtered out the more specifics, the traffic fell on the floor, because 
it was sent to a location that was announcing the supernet that 
had no reachability to the correct destination. 

Their assumption that *everyone* would hear the more specifics, 
and thus the traffic would flow to the right island location was the 
"failure to understand BGP" that I was commenting on, and noting 
that while it is entirely correct to decide if you want to filter prefixes 
of an arbitrary length from entering your network, you may discover 
in the process that other networks that do not understand BGP and 
routing in general may complain that you have Broken The Internet(tm)
by doing so.

Assuming that your announcement of more specifics will always pull 
traffic away from a less-specific announcement is overly-optimistic.
While it may *often* work, you should still be prepared to deal with 
traffic arriving at your least-specific announcement as well.
  
This turned out to be something that not every network on the
Internet fully grasps, and my original message was warning that 
filtering on /24s would potentially bring complaints from networks 
like those.

It took a roundabout path, but I'm glad we eventually both ended 
up at the same place.   :)

Thanks!

Matt