Long AS Path

Pierfrancesco Caci pf at tippete.net
Thu Jun 22 06:09:41 CST 2017

>>>>> "Mel" == Mel Beckman <mel at beckman.org> writes:

    Mel> Why not ask the operator why they are pretending this path? Perhaps
    Mel> they have a good explanation that you haven't thought of. Blindly
    Mel> limiting otherwise legal path lengths is not a defensible practice, in
    Mel> my opinion.

    Mel>  -mel beckman

A prepend like that is usually the result of someone using the IOS
syntax on a XR or Junos router.

Long ago, someone accidentally prepending 255 times hit a bug (or was it
a too strict bgp implementation? I don't remember) resulting in several
networks across the globe dropping neighbors. One has to protect against
these things somehow. 

As a data point, here is how many prefixes I see on my network for each
as-path length, after removing prepends:

aspath length	count
	0:	340
	1:	47522
	2:	292879
	3:	227822
	4:	58390
	5:	10217
	6:	2123
	7:	638
	8:	48
	9:	58
	11:	20
	12:	2

So, does your customer have a legitimate reason to prepend more than 5
times? Maybe. I still think that anyone that does should have their BGP
driving licence revoked, though.


Pierfrancesco Caci, ik5pvx

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