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
On Tue, 19 Sep 2017 13:33:03 -0000, craig washington said:
How many AS PATHS are too many?
Well - how many do you see when things are operating nominally? How many do you regard as "the other end is obviously too crazy to listen to"? Add them up and divide by two. Of course, the hard part is quantifying those two values - the network engineers for the AS I work for probably have a different tolerance level for such shenanigans than the guys running a Tier 1/1.5/more-than-2 network (and *those* guys almost certainly have different tolerances based on which of their peers and transits they're talking to)....
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
In my MUCH younger days, I may have helped abuse the global table via prepends, but never to that level :) On Wed, Sep 20, 2017 at 4:36 PM, Randy Bush <randy@psg.com> wrote:
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.
and they are probably still wondering why it does not achieve what they want.
randy
Too many prepends = any more than you really need for what you're trying to accomplish. :) I've cutoff paths as short as 4 to as long as 8 before in different jobs for different reasons. On Tue, Sep 19, 2017 at 9:33 AM, craig washington < craigwashington01@hotmail.com> 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
Thank you all very much for the feedback. As always it is much appreciated. ________________________________ From: Tom Beecher <beecher@beecher.cc> Sent: Wednesday, September 20, 2017 8:01 PM To: craig washington Cc: nanog@nanog.org Subject: Re: AS PATH limits Too many prepends = any more than you really need for what you're trying to accomplish. :) I've cutoff paths as short as 4 to as long as 8 before in different jobs for different reasons. On Tue, Sep 19, 2017 at 9:33 AM, craig washington <craigwashington01@hotmail.com<mailto:craigwashington01@hotmail.com>> 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
participants (6)
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craig washington -
jim deleskie -
Randy Bush -
Tim Evens -
Tom Beecher -
valdis.kletnieks＠vt.edu