Re: Why MSDP?

[David Meyer <dmm@cisco.com>]

	Not sure about the history of this thread, but here's some
	of the history.

	First, we looked at putting SAs into BGP. I wanted to
	do this as a first thought since the MBGP/MSDP split
	effectively splits the control plane. However, this 
	turned out to be a less than optimal solution for several
	reasons, not the least of which included:


	(i).	State

		The reason MSDP is periodic and not incremental
		(and originally non-caching) is that we were tryin
		to avoid explosion of (S,G) state. I guess we're
		not so worried about that (c.f. SSM). In addition,
		we had to somehow (attempt to) solve the bursty 
		source problem.

	(ii).	BGP Stability

		I don't really want to argue that the dynamic nature
		of SA advertisements is going to help the stability
		of BGP (that is, of the global routing system). Neither
		do you, I would guess.

	(iii).	Deployment

		Because of (i). and (ii). above, SAs in BGP would have been
		impossible to deploy (who would want to deploy a 
		version of BGP that carried SAs? MBGP with SAFI \in {1,2} 
		was hard enough).

	
	There were a few other things, but these were the main concerns.
	BTW, what we were doing was attempting to find a way to free
	providers from having to co-locate their RPs on a dense-mode
	exchange point (i.e., get (S,G) state from outside their domain onto
	their RPs so their customers could join those groups, w/o sharing
	RPs). That was the (lost in the mist of history) design goal.

	Dave

		
According to Ali Boudani:
> 
> > One reason: the timescales for change of active source indications are
> > much different than BGP was designed to carry.  BGP wants to carry
> > data that doesn't change very often, e.g. see route dampening.  Sources
> > can come and go at an arbitrary rate, so the rate of change of the
> > information is potentially much higher.
> 
> can you specifie more,
> why should the timescales for change active source indications are much
> different.???
> 
>