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Re: Why doesn't BGP...

  • From: Marten Terpstra
  • Date: Sat Nov 09 13:48:00 1996

Ed Morin <edm@halcyon.com> writes

 * > The first one is easy, in fact you can do that yourself by fiddling
 * > with metrics or such on the different BGP sessions. The second one
 * > would have dramatic consequences in terms of route instability. You
 * > pick one route now because of load on the link, the load changes and
 * > you pick the other, now BGP will have to change the announcement of
 * > this network to other peers. So, now we not only have flaps because of
 * > links/routers going up and down, we also have flap because of load
 * > changes on the network. The result: you are dampened out forever, or
 * > the network falls over.
 * 
 * Is this really true?  All I'm asking for is that the route a router
 * considers to be "best" be picked by something a little more rational
 * than the ordinate order of its IP address relative to another link.
 * I don't see a flap situation at all here -- only that a decision to
 * route a packet may change more frequently based on load.

Ed,

you have to think about this in generic terms. In your network you may
not re-advertise BGP info you get from your peers. In most networks
these routes do get re-advertised. On of the basic rules in BGP is
that you only advertise routes you actually use. Following that rule,
if you change your decision on the best route based on load, you have
to change your advertisement of that route to all you announced it
to. The result is a flap.

-Marten
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