Tulip Rasputin wrote:
Do the ISPs ever look for some particular AS number in the BGP
AS_PATH and then decide what action/preference/priority they
need to take/give based on the AS number(s) present in the BGP
AS_PATH_SEQ/SET?
If there is a question that nobody here wants to answer, this would be
it.
Short answer:
Publicly: No, heaven forbid. I'll just let BGP do its job.
Privately: Hell yes.
Michel.
I'm not a router guy (routing atleast), but perhaps there are performance
problems inside an ASN along a path which you connect to other places? So
you might lengthen paths through/to that ASN to force traffic across
another ASN's direct connection which is less problematic?
Or, you just don't want to send traffic through Bill Manning's ASN because
you dislike his hawiian T-Shirt Policy? There are probably a few hundred
reasosn why you'd avoid an ASN... In general though I'd think that like
Michel said: "It's a pain and its doing something that bgp should do for
you without lots of messing about"
Hi Chris,
Or, you just don't want to send traffic through Bill Manning's ASN because
you dislike his hawiian T-Shirt Policy? There are probably a few hundred
reasosn why you'd avoid an ASN... In general though I'd think that like
Michel said: "It's a pain and its doing something that bgp should do for
you without lots of messing about"
That's why i explicitly asked for some "social/political/etc." reasons where an ISP may not want his traffic to traverse some particular AS number(s). Something which is beyond BGP to determine as of now ! 
I believe with the responses that i received both on the list and offline, that it is indeed quite normal for ISPs to filter routes based on the AS Paths for 'other' reasons. Reasons, quite beyond BGP as a protocol to handle! And this can happen, when an ISP doesnt want its traffic to traverse some AS(es).
Thanks,
Tulip
IIRC, at some long ago time, there was a Canadian policy, derived from a policy on telco transit, that two Canadian providers could not use transit through the USA to get to one another. This is long gone.
I can't say if it is still the case, but, at one point, the PRC would drop routes that used Taiwan as transit.
choosing a route whose AS path doesn't contain an AS is no garantee
that packets won't flow through that AS nevertheless. Forwarding is
being done hop-by-hop. Examples for that szenario are
* more specifics that aren't distributed globally
* broken/complicated routing setups where a router chooses a route it got
via IBGP and where on the path to the corresponding nexthop another
router decides to rather use his own route (e.g. a router chooses his
own route due to step "external vs. internal" in the BGP decision process,
but the original decision was based on lowest router id) This can happen
in not-full-mesh environments, e.g. with route reflector setups...
I use the AS-path rather to grab chunks of prefixes (e.g. all routes
via ASxyz) to do loadbalancing over transit providers whose announcements
look so similar that the decision is only based on lowest router id of the
IBGP speaker. I know this can be "somewhat solved" via multi-path bgp but
there are other issues with that (fib size, multiple exit points behind
one route-reflector etc)...
tschuess
Stefan