router startup behavior
Borchers, Mark
mborchers at splitrock.net
Tue Jan 15 14:23:03 UTC 2002
> the most likely cause would be one of:
<items deleted for brevity>
> (c) script used to configure router(s) adds a 'network'
> statement prior to trimming route-filters
Yeah, (c) seems most likely to me. Ratul, a script like
this or some variant could cause what you are seeing:
config-router# no neighbor <a>
config-router# no neighbor <b>
config-router# no neighbor <c>
(script to rewrite filters executes)
config-router# neighbor <a> remote-as <x>
config-router# neighbor <a> remote-as <y>
config-router# neighbor <a> remote-as <z>
(sessions start coming up)
config-router# neighbor <a> route-map <A> out
config-router# neighbor <b> route-map <B> out
config-router# neighbor <c> route-map <C> out
config-router# Ctrl-Z
# clear ip bgp external soft out
Just guessing - you're seeing these events between midnight
and 5 am?
> At 01:10 PM 14/01/2002 -0800, Ratul Mahajan wrote:
>
>
> >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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