[195119] in North American Network Operators' Group
Re: Long AS Path
daemon@ATHENA.MIT.EDU (Steve Lalonde)
Thu Jun 22 11:15:41 2017
X-Original-To: nanog@nanog.org
From: Steve Lalonde <steve@enta.net>
In-Reply-To: <E5A2DC12-0F5E-4669-983F-5807EECBCA47@beckman.org>
Date: Wed, 21 Jun 2017 17:50:27 +0100
To: Mel Beckman <mel@beckman.org>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
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@beckman.org> wrote:
>=20
> Steinar,
>=20
> 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.=20
>=20
> 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.
>=20
> -mel beckman
>=20
> On Jun 21, 2017, at 12:57 AM, "sthaug@nethelp.no" <sthaug@nethelp.no> =
wrote:
>=20
>>> Just wondering if anyone else saw this yesterday afternoon ?
>>>=20
>>> Jun 20 16:57:29:E:BGP: =46rom Peer 38.X.X.X received Long AS_PATH=3D3D=
AS_SEQ(2=3D
>>> ) 174 12956 23456 23456 23456 23456 23456 23456 23456 23456 23456 =
23456 234=3D
>>> 56 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 23456 =
23456 =3D
>>> 23456 23456 23456 23456 23456 ... attribute length (567) More than =
configur=3D
>>> ed MAXAS-LIMIT
>>=20
>> There are quite a few examples of people using stupidly long AS =
paths.
>> For instance
>>=20
>> 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
>>=20
>> I currently have 27 prefixes in my Internet routing table with 40 or
>> more ASes in the AS path (show route aspath-regex ".{40,}").
>>=20
>> 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?
>>=20
>> Steinar Haug, Nethelp consulting, sthaug@nethelp.no
