Long AS Path

Wed Jun 21 16:50:27 UTC 2017

Mel,

There was a Cisco bug many years ago that caused lots of issues. Since then we have limited max-as to 50 and it has not caused any reported issues yet.

Link that does not require a CCO login to view.
http://blog.ipspace.net/2009/02/oversized-as-paths-cisco-ios-bug.html

Regards

Steve Lalonde

> On 21 Jun 2017, at 16:49, Mel Beckman <mel at beckman.org> wrote:
> 
> Steinar,
> 
> What reason is there to filter them? They are not a significant fraction of BGP paths. They cause no harm. It's just your sense of tidiness. 
> 
> You might consider contacting one of the operators to see if they do have a good reason you haven't considered. But absent a good reason *to* filter them, I would let BGP mechanics work as intended.
> 
> -mel beckman
> 
> On Jun 21, 2017, at 12:57 AM, "sthaug at nethelp.no" <sthaug at nethelp.no> wrote:
> 
>>> Just wondering if anyone else saw this yesterday afternoon ?
>>> 
>>> Jun 20 16:57:29:E:BGP: From Peer 38.X.X.X received Long AS_PATH=3D AS_SEQ(2=
>>> ) 174 12956 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 234=
>>> 56 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 =
>>> 23456 23456 23456 23456 23456 ... attribute length (567) More than configur=
>>> ed MAXAS-LIMIT
>> 
>> There are quite a few examples of people using stupidly long AS paths.
>> For instance
>> 
>> 177.23.232.0/24    *[BGP/170] 00:52:40, MED 0, localpref 105
>>                     AS path: 6939 16735 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 28163 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262401 262949 52938 52938 52938 52938 52938 52938 52938 52938 52938 52938 52938 I
>> 
>> I currently have 27 prefixes in my Internet routing table with 40 or
>> more ASes in the AS path (show route aspath-regex ".{40,}").
>> 
>> I see no valid reason for such long AS paths. Time to update filters
>> here. I'm tempted to set the cutoff at 30 - can anybody see a good
>> reason to permit longer AS paths?
>> 
>> Steinar Haug, Nethelp consulting, sthaug at nethelp.no