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Re: router startup behavior

  • From: Lincoln Dale
  • Date: Tue Jan 15 03:03:59 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 stress/oscillation)
(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 live-production-



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