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
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
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
In my MUCH younger days, I may have helped abuse the global table via
prepends, but never to that level 
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.
Thank you all very much for the feedback.
As always it is much appreciated.