[c-nsp] Asymmetric routing

[Gert Doering]

Hi,

On Thu, May 11, 2006 at 11:33:12AM +1000, Martin Barry wrote:
> $quoted_author = "Michael Robson" ;
> > Has anyone experienced asymmetric routing within their network
> > that caused multicast RPF checks to fail and so break multicast?
> > More generally, what are people doing design-wise on their networks
> > , if anything, to avoid such problems on ever increasingly resilient,
> > meshed networks?
> 
> surely RPF is only being applied at the edge of your AS?

Michael was asking about *multicast* RPF, which is a completely different
thing, and not actively "applied" anywhere, but part of multicast routing.

Unfortunately I don't have a definite answer myself.

My gut feeling is that for the multicast routing *inside* the AS, m-RPF
doesn't mind asymmetric routing, as packets are flowing only one way 
anyway, and the flow of packets is based on PIM JOINs, that are 
themselves based on the reverse path tree.

For external routing to and from other ASes, and MSDP announcements, it
is a bit more tricky, and I haven't fully though through all details
yet - but still, due to the way MSDP and PIM decide where to "route"
packets (more specific: where to send PIM JOINs to), it *should* be
no problem.

No idea about SSM or bidir-PIM, though.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de