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Re: router startup behavior
From: Ratul Mahajan <ratul () cs washington edu>
Date: Mon, 14 Jan 2002 13:10:23 -0800 (PST)



to the best of my knowledge, here is what is happening.

1. router starts rebooting
2. there are routes in the routing table, some of which are not to 
be announce according to filters
3. bgp sessions comes up; the filters have not yet taken effect
4. start announcing routes
5. filters come up
6. the router realizes that it made a mistake and withdraws the routes not 
meant to be announced.

i should also point out that all such incident are not 1000 router. most 
of them are 20-50, but i have seen non-trivial number of ~100 prefixes, 
and a couple more than that.

        -- ratul

On Mon, 14 Jan 2002, Ratul Mahajan wrote:



at university of washington, we are doing a measurement study of bgp
misconfiguration
(http://www.cs.washington.edu/homes/ratul/bgp/index.html).  

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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