Routing flaps, was Re: Ping flooding

Justin W. Newton justin at
Fri Jul 12 18:43:17 UTC 1996

At 10:37 AM 7/12/96 -0400, Paul Ferguson wrote:
>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 
>>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.

I think the real difference is this.  If my routes are only seen at one
place then it makes sense for me to announce them whether or not I can
actually get to them.  If, however, it is possible to get to parts of my
network from one exchange, but not another then the routes should only be
announced where they can actually be gotten to from (obviously).  I.e. if I
am a small to mid-sized provider who has multiple t-1's to the net to
different providers, and then those  t's come into the same physical network
(either the same router or one's that are topologically adjacent to one
another) chances are good that if one of those routers can't get to part of
my network the other probably can't either.  In this instance it makes sense
for me to null0 the routes and do my part to not affect instability (only
routes which originate in my AS though, if I have any multihomed downstreams
then they would not want me announcing their routes when I can't get to
them).  If, however, I have a network which has multiple egresses to the
internet at large it is very possible for one part of my network not to be
able to reach certain sites while other parts can.  I would then want to
"announce my instability" so that I am only announcing networks where they
can actually be reached from.

Justin "Long babbling posts R us" Newton

Justin Newton
Internet Architect
Erol's Internet Services

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