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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: weird BGP cisco-ism? [problem resolved]
Well, that was a week-long disruption in routing that shouldn't have been able to happen. This is an explaination of the problem I described last night... We have a handful of CIDR blocks that we announce, but one of them was getting flap-damped. By writing a tool to monitor the BGP announcements, we discovered that a reannouncement was happening every few minutes, for no apparent cause. What was happening is that, about a week ago, another provider got a request from a customer to route them a single /24 from the middle of our (non-portable) block. Because there was no IGP route for that customer, but a network line was in place, the other provider was, every 5-20 minutes, announcing the route for about 30 seconds, and then withdrawing it. We peer with that provider directly, and can also see them via our transit provider, and we have "summary-only" set in our BGP configuration. Every time the /24 appeared, our router would send *four* repeated announcements of the /20 block... once when it appeared via the direct session, again when it appeared via our transit provider, and then twice more, once for each withdrawl. This then triggered the flap damping code at our peers, causing them to ignore the /20 route altogether. Problem took forever to find because the only way to catch it was to know exactly when the flaps were happening and then, at that very moment, look for any longer prefixes in the table. Any suggestion as to how to prevent my Cisco from saying *anything* when someone else decides to briefly present a more specific route would be appreciated, and yet again, we'd be better off with some sort of authentication as to who can advertise what to whom. -matthew kaufman [email protected]
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