Routing flaps, was Re: Ping flooding

Forrest W. Christian forrestc at
Fri Jul 12 16:17:11 UTC 1996

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 at

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