Hi Guys...
i have a pretty good understanding in multicast technologies, but today i ran into some doubts...
The thing is:
- When we have sparse-dense mode configured and the router does NOT know a RP for a particular group, it would fall into dense mode if the "ip pim dm-fallback" command is in place. No doubts here.
now... imagine the followind topology:
r1 -- r2 -- r3
all routers with multicast enabled and sparse mode on each interfaces, including loopbacks.
We have no auto rp, static rp, bootstrap.... anything, no INDICATION of RP on the routers.
R3 joined the MCAST group of 225.1.1.1 (int loop0 / ip pim join-group 225.1.1.1)
- R1 can NOT ping 225.1.1.1, as its NOT supposed to, as the router does NOT know about the RP
- R2 CAN pint 225.1.1.1, and looking at the mroute table on R3, it says it has the DENSE mode flag and also the SPT flag on it.
so... why R2 can run it on dense mode, having just sparse mode configured ?
some cisco DOcumentation says that SPARSE mode FALL back to DENSE if does not know the RP. i THOUGHT this was just the case of SPARSE-DENSE mode, but apparently for direct connected receivers SPARSE mode runs on DENSE.
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_20_se/configuration/guide/swmcast.html#wp1077573
(under auto-rp section: If no statically configured RP exists, the router or switch changes the group to dense-mode operation.)
need som e clarification on how this works exactly, its pretty much acting as a broadcast in my opnion!!
thanks in advance
