Lots of prepends - AS20912 case

Jon Lewis jlewis at lewis.org
Fri Feb 20 13:00:46 UTC 2009

On Fri, 20 Feb 2009, Giuliano Peritore wrote:

>        The problem is that differently to Cisco the syntax of the prepend 
> field on thius system is not a string (eg. "20912 20912 20912") but an 
> integer, that the user interface _should_ limit to the interval 0-16.
>        The producer has been warned about the problem, which I can't 
> completely define as a "bug"... but the lack of a user configuration helper 
> (syntax checker).

More important than whether or not to consider this a bug, it seems a very 
shortsighted way to support prepending.  If your prepend "field" is an 
integer controlling how many times to prepend, how do you control which 
ASN(s) or even AS Paths are prepended?  It sounds like you probably can't. 
As has been discussed recently, there are cases where you might want to 
prepend a creative AS Path for traffic engineering purposes to force 
certain routes/paths to be ignored by certain ASNs.

  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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