AS path question.

Wed Nov 10 20:31:25 UTC 2010

On Nov 10, 2010, at 3:22 PM, Greg Whynott wrote:

> 
> 
> Recently I adjusted the maxas-limit option on our router,    logs started reporting routes being refused because the AS path is to long.   seems to work as expected.
> 
> when I looked at the logs I was a bit confused at what i was looking at...   why is it there are multiple AS's in the path that appear to be the same AS?  I expected an AS path comprised of mostly unique ASs.
> 
> instead of this:
> 
> 476330: Nov 10 14:55:07.247 EDT: %BGP-6-ASPATH: Long AS path 549 26677 6939 21011 43022 43022 43022 43022 43022 47359 47359 47359 47359 47359 47359 47359 47359 received from isp router: More than configured MAXAS-LIMIT
> 
> 
> 
> i expected it would look more like:
> 
> 476330: Nov 10 14:55:07.247 EDT: %BGP-6-ASPATH: Long AS path 549 26677 6939 21011 43022  47359 received from … .. .

People prepend, and think 'more is better' vs using communities and other 'complex' methods of managing their traffic.

It's also the easy tool from the shed.  'set as-path prepend blah' is easier than

match blah, set community blah, match something else, set community blah2, match something3, set something3

in the typical cisco parlance.  It's perhaps better (or worse) depending on your vendor and how the policies are actually interpreted and how granular you need to be.

The best question is:

Do you know what prefix you just lost reachability to, or do you just point default as a last resort anyways, so don't know.

- Jared