AS path question.

Jon Lewis jlewis at
Wed Nov 10 20:35:50 CST 2010

On Wed, 10 Nov 2010, Scott Weeks wrote:

> Why did that make you feel safe?  Other than a bug, and ignorance of 
> BGP, what is unsafe about a lotta prepends?

Ignorance of BGP?  There's a known cisco bug that causes BGP session 
resets when as as-path length exceeds 255.  I've been running with bgp 
maxas-limit 75 for years as a "just in case there are other bugs & I find 
it very hard to believe anyone legitimately needs an as-path length 
anywhere near that long".  Worst case, someone is silly with their number 
of prepends, we don't see their route.  I can't say how long I've been 
doing predates our rancid setup, which means >6 years.  Though 
it's caused numerous dropped routes, it hasn't generated a single 

In your opinion, is filtering of BGP routes based on prefix length also a 
sign of ignorance?  Everyone should just be letting all the crap through?

  Jon Lewis, MCP :)           |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
_________ for PGP public key_________

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