Re: Routing flaps, was Re: Ping flooding
At 07:40 AM 7/12/96 -0600, Forrest W. Christian wrote:
However, the world never needs to see this - it's an internal routing issue and when the internal route flaps it shouldn't affect the external routes.
So in summary:
1) Never flap your routes in public. What you do in your own home is your own business.
I'm sorry, but I'm afraid that you're missing the point here. In most larger ISP backbones, the behavior of their IGP is indeed visible to the public, since in most instances, most of the Internet traffic relies on the stability of these interior (an esoteric term) networks. Therefore, whether interior or exterior flap is really of no relevance in this context. In smaller ISPs, where not a great deal of public traffic is transiting their (interior) backbone, granted, it is of lesser importance and visibility. - paul
On Fri, 12 Jul 1996, Paul Ferguson wrote:
I'm sorry, but I'm afraid that you're missing the point here.
In most larger ISP backbones, the behavior of their IGP is indeed visible to the public, since in most instances, most of the Internet traffic relies on the stability of these interior (an esoteric term) networks. Therefore, whether interior or exterior flap is really of no relevance in this context.
<This isn't intended to irritate, I just want to make sure I understand this correctly> Let me try a very simple example: To Internet To Internet <-- Peering points or upstreams 'Point A' 'Point B' | | +-----------+ +-----------+ | Router A |---| Router B | +-----------+ +-----------+ | | | | +-----------+ +-----------+ | Router C | | Router D | +-----------+ +-----------+ | | To Downstream To Downstream Non-BGP (Static) Non-BGP (Static) Customer Customer Routers A & B are running BGP to the outside world, and iBGP between them. Routers C & D are 'defaulted' into A and B. If the link between A&B dies the Exterior Routes will (and should) flap. If the link between either A or B and the Internet dies, the Exterior Routes will (and should) flap. 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. I think that's what I meant to say before. Sometimes I'm not too clear about what I'm saying. If I've still missed the boat feel free to let me know :) -forrestc@imach.com
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
Forgot to throw one more item in here... On Fri, 12 Jul 1996, Paul Ferguson wrote:
I'm sorry, but I'm afraid that you're missing the point here.
In most larger ISP backbones, the behavior of their IGP is indeed visible to the public, since in most instances, most of the Internet traffic relies on the stability of these interior (an esoteric term) networks. Therefore, whether interior or exterior flap is really of no relevance in this context.
In smaller ISPs, where not a great deal of public traffic is transiting their (interior) backbone, granted, it is of lesser importance and visibility.
What I forgot to say is that I fully agree with what you are saying here- I doubt that a large provider such as Sprint or MCI or Agis or any other major internet backbone is going to be able to hide much of their internal route flapping, as their internal routes ARE their external routes. My original note was intended to reply to the specific case of using dynamic routing to distribute routes which could just as easily be done with static routes - I.E. where there is one and only one path to the destination. In this case, it is sometimes desirable and necessary to run some sort of interior routing protocol, but it is not desirable that flaps caused by a break along the one and only one path be propogated to the outside world, but instead the packets should be null0'd at the point of path convergence. -forrestc@imach.com
participants (3)
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Craig A. Huegen -
Forrest W. Christian -
Paul Ferguson