Current filters for BGP announcements

Darren Bolding darren at
Fri Jul 30 21:54:58 UTC 1999

Hello all,

I'm sure most, if not all of you are familiar with the filters that various
ISP's and NSP's have placed on received BGP announcements.  Most famous
for this is/was? SprintLinks filter policy, which is documented at

Recently the subject of announcement filters and route-dampening policy has
become increasing relevant to a number of my customers, especially when
making sound architecture decisions.

Unfortunately, few providers have documented their filter policies (to my
knowledge) as well as SprintLink.  I'm not even sure Sprint follows the 
policy anymore.  I have specific knowledge of several cases where they are
listening to /24's announced by some companies (albeit these are very high
profile, and would easily be justified as exceptions to the rule)

My question to you, and I believe NANOG to be the most appropriate forum
I know of in this regard, is the perenial "what is the current general policy"
in this regard.

I can think of a few ways to get limitted knowledge about providers policies
by checking for received-announcements on edge routers/looking glasses which
receive transit from other providers (e.g. go to an MCI customer and look
for short prefix announcements known to be announced at, say, Mae-East).

Unfortunately, this is not particularly sound, and is high-effort.  Further,
it does nothing to address the second question of mine, which is what the 
current typical route-dampening policies are.

Third, and this isn't North-American related, so please refer me to better 
sources, should they exist, are these policies any different for other 
geographic areas?  (e.g. will people listen to smaller announcements at 
international peering points, do people place different filters for APNIC
address space, etc.)

Finally, what is the typical reconvergence time for BGP in the global Internet

For full disclosure, my company makes high availability and load balancing 
products, and I would certainly prefer these questions be answered in certain
ways.  I am, however, most interested in the truth so I can give educated
advice to my customers.  It seems like it is no longer reasonable to keep
constant track of who is doing what with routing policies, as it isn't a
simple matter of keeping state on 4-8 providers.  At least not unless driving
BGP sessions is your day-to-day job.  Currently, my best answers (qualified
with "I may be out of date" are

(1) Varies, but a /19 is probably safe, any smaller likely depends on who
	you buy transit from.

(2) Default dampening settings.

(3) I don't remember, several ideas were kicked around.

(4) 15-20 minutes, depending on lots of variables.

Thanks for everyones input on this, any pointers are greatly appreciated.


--  Darren Bolding                                 darren at   --
--  Principal Consultant					   --

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