router startup behavior

Lincoln Dale ltd at
Tue Jan 15 07:52:44 UTC 2002


"route-filtering", "BGP" and "route announcement" all go hand-in-hand.
all are control-plane functions.

for router-vendors that matter, i doubt that the behavior you describe occurs.

the most likely cause would be one of:
  (a) a bug.  (but if it is there, i'm surprised it isn't causing more 
  (b) people changing route-policy.  ie. it isn't "router starting up" but 
more likely
      someone going "route-map FOO deny 20"; "no match ...", "match ...".
  (c) script used to configure router(s) adds a 'network' statement prior 
to trimming
  (d) too many people experimenting with route-injectors on that 



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
> > (
> >
> > 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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