On Fri, 12 Jul 1996, Forrest W. Christian wrote: ==>If the line between routers A&C or B&D or between either C or D and their ==>respective static downstreams die, there should be NO external route ==>flap. However, if C&D are incapable of 'Null0' routing, it may be ==>beneficial to run dynamic routing between A&C and between B&D so that ==>A&B discard packets instead of causing a routing loop. This "internal ==>routing flap" should not be visible to the outside world. It can be, though. Or at least from what I've seen. If you have a 'floating static' to null0, it won't take over until the dynamic holddown timer expires (whatever it happens to be for the particular IGP you're using). For example, a situation like this: Router A <----> Router B <----> TermServ (with ISDN)<--DIAL-UP-LINK-->Router C Router A is the access-point router doing BGP. The customer on router C has a set of addresses assigned "way-back-when" by the InterNIC, and router A is advertising that set via BGP. Routing is done between A & B using OSPF. The LAN between B & TermServ is running RIP (because certain manufacturers' boxen can't do anything BUT RIP). The organization is designed in such a way that their static dial-up address is portable among all POPs in their provider (so the B & TermServ could be in any of their POPs). Router A has a static route to null0 to hold the BGP route in place. When router C is dialed up and present, the route gets propogated from TermServ to B via RIP, which redists into the OSPF area. Router A picks up this route via OSPF. The dynamically-learned route for customer 'C' now takes precedence over the static route to null0 on A. If for some reason, C drops the dial-in links and the TermServ goes through a few RIP update cycles, the route will be marked as 'inaccessible', but will still be in the routing table until hold-down expires. When this route is marked as 'inaccessible', the static route does not take precedence and the route is withdrawn from BGP--and consequently re-introduced when the hold-down expires and the null0 route takes precedence. Now, if there's something in the config of router A that can be used that will prevent this flapping, or if it was just a freak coincidence that routers saw a flap after dropping the connection, then please correct me. /cah ---- Craig A. Huegen || || Network Analyst, IS-Network/Telecom || || cisco Systems, Inc., 250 West Tasman Drive |||| |||| San Jose, CA 95134, (408) 526-8104 ..:||||||:..:||||||:.. email: chuegen@cisco.com c i s c o S y s t e m s