router startup behavior

Steve Naslund snaslund at
Mon Jan 14 19:28:42 UTC 2002

Here is my best guess as to what you are seeing.  Most likely a large CIDR
block is announced
by a service provider A.  A small CIDR block is given to a customer who is
connected to multiple
service providers and thus running BGP.  Now the more specific route is
announced by service provider B,
he does not own the block but is announcing it on behalf of service provider
As customer.  What is happening is that the customer has a line or router
failure and that withdraws their more specific announcement from service
provider B.  Since the service provider A is announcing a supernet route he
now becomes the only route
for that block.

Steven Naslund
Network Engineering Manager - Chicago

> -----Original Message-----
> From: owner-nanog at [mailto:owner-nanog at]On Behalf Of
> Ratul Mahajan
> Sent: Monday, January 14, 2002 12:54 PM
> To: nanog at
> Subject: router startup behavior
> at university of washington, we are doing a measurement study of bgp
> misconfiguration
> (
> one of the things we found is that there are a lot of announcements of
> more-specifics that come and go within a matter of 2-5 minutes.
> by talking to the operators involved in these incidents, we found that
> most of these are caused when the router is rebooted (intentionally or
> not). while some operators were aware of this side effect, most were not,
> and were taken by surprise that they just injected anywhere from 1-1000
> routes into BGP only to withdraw them a couple of minutes later.
> i would like to understand this behavior better. is this behavior
> vendor-specific (cisco?) or pervasive? is there a configuration style that
> causes or avoids this "spill-over"?
> my understanding is limited to this happens when the bgp session comes up
> too soon, before the filters have taken effect. could someone familiar
> with router internals shed some light on it?
> the problem is limited to route origination only, or also propagation?
> in other words, can a router propagate a route it should not while
> starting up because export filters are not yet in place?
> never ever gotten my hands dirty into router configuration; your input
> would be invaluable.
> thanks,
> 	-- ratul

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