> > >> So this is to choose between identical routes. Why is this needed? > > I have no idea. You'll have to ask Pierre. > > (And I'd appreciate an explanation myself.) >
*clearing throat* :) Here is my humble understanding as a multicast non-expert. PIM makes an extensive use of RPF (Reverse Path Forwarding) to build the multicast forwarding tree. RPF can be done with the routing table alone, with no metric. You just need to know the ‘upstream’ interface to either an RP address, or a source address. The issue comes from that multicast routing is not about sending a packet to a next hop, but to a next link. This situation implies that you may have multiple routers that, according to the RPF and PIM Join/Prune, could be candidate to forwarding a packet to/from some link. When such a situation occurs, an election mechanism is used to solve it. - In PIM-SM, it is not reactively using Asserts - In PIM-BIDIR, it is not proactively using DF election mechanism. Both of these techniques makes use of a metric in order to decide which one is the ‘best’ router to forward some packet. I do not know exactly what are the consequences for PIM-SM if you don’t have these metrics (Asserts are won randomly). But I think that in PIM-BIDIR, you can end-up with stable routing loops. - Pierre _______________________________________________ homenet mailing list homenet@ietf.org https://www.ietf.org/mailman/listinfo/homenet
