[195946] in North American Network Operators' Group
Re: AS PATH limits
daemon@ATHENA.MIT.EDU (Tim Evens)
Sat Sep 23 03:37:49 2017
X-Original-To: nanog@nanog.org
Date: Wed, 20 Sep 2017 10:34:39 -0700
From: Tim Evens <tim@snas.io>
To: nanog@nanog.org
Mail-Reply-To: tim@snas.io
In-Reply-To: <SN1PR0701MB206268455259CC64FC0D8ED7A8600@SN1PR0701MB2062.namprd07.prod.outlook.com>
Reply-To: tim@snas.io
Errors-To: nanog-bounces@nanog.org
An AS_PATH is encoded with one or more segments. Each segment has a
maximum size of 255 entries (8 bit segment length). The absolute limit
will depend on the complete BGP message size, which is limited to 4096
and extended via draft-ietf-idr-bgp-extended-messages.
The longest as_path at this time (changes frequently though) is 51
entries, but in the past we have seen as many as 501.
Below is an example showing an excessive amount of prepending for prefix
185.135.134.0/23 at 2017-09-18 20:20:05 UTC.
as_path_count: 501
as_path: 38726 9957 17604 7922 6830 197451 197451 197451 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
207239 207239 207239 207239 207239 207239 207239 207239 207239 207239
201228
--Tim
On 19.09.2017 06:33, craig washington wrote:
> Hello world.
>
> I was wondering and forgive me if this discussions has already taken place.
>
> How many AS PATHS are too many?
>
> Meaning how do we determine how many to filter on transit links or public peering links?
>
> Thanks in advance
