4 Byte AS tested

Geoff Huston gih at apnic.net
Thu Jan 11 20:14:49 UTC 2007

At 04:59 AM 12/01/2007, Todd Underwood wrote:

>we (renesys) saw as23456 adjacent to both 1221 (expected) and 65001
>(not), originating two prefixes:

that was me, yes :-)

I apologise for the 65001 leak . In mitigation I can only add that it 
did not last very long!

>paths looked like:
><peer> 7474 1221 65001 23456 23456 23456
>and many similar
>but also
><peer> ... 4637 1221 23456
>and many similar
>was the leak of the 65001 as intentional and part of the experiment, a
>simple, error, or is there something useful to learn about the
>difficulties of building filter lists with 4 byte ases?

At the time I needed a 2 byte AS between the OpenBDPD tester and 
AS1221 and I thought it was perhaps less silly to leak a private use 
AS than it was to steal a non-private use AS.

Building filter lists in the 2 byte world to filter out 4 byte paths 
is an issue, as all the 4 byte entries in the path are translated 
into AS23456 when you are in the 2 byte world. This could get tricky 
if you have a complex routing policy that you want to implement and 
some of your policy targets are using 4 byte AS numbers.



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