Subject: Re: A modest proposal: get rid of (*,*,RP) state

From: Dave Thaler <[email-masked]>
Date: Mon, 12 Jun 2000 10:57:57 -0700 (PDT)

Pavlin asks:
> I am probably missing something in the picture, but why do we need
> (*,*,RP) if the RP is not a border router?
> To "push" the data from the internal sources over the BGMP trees to
> the dense-mode domains? (again, Domain-Wide Reports running in the
> dense-mode domains will eliminate the need for this).
> 
> A more detailed example here would be helpful :)

Technically one would only need a bunch of (*,Gprefix,RP) joins per RP
but those aren't specified, so a single (*,*,RP) per RP works :)

The exit router for Gprefix (i.e. the one towards the root domain
for Gprefix) needs to receive all data from the subtree under it
for all groups G in Gprefix.  If it's the RP for all such groups, 
there's no problem, all packets travelling uptree automatically
get sent to it.

If it's not the RP, it needs a way to join to all RPs that serve
groups in Gprefix.  (And any other Gprefix'es for which it is
the best exit router.)  DWRs don't help here, since this is a border
router needing to set up a path to an internal RP.

So to repeat, solutions I'm aware of are:
1) Don't run PIM-SM... run BGMP as the M-IGP (as Dino hinted, I think)
2) Run PIM-SM with RPs=BRs.
3) Run PIM-SM with MSDP for internal peerings only, between RPs & BRs.
4) Run PIM-SM with (*,*,RP) joins from exit BRs to RPs.

-Dave