Objectives
The simulator ticks each one as soon as the network meets it.
Ping 239.1.1.1 from SRC and read why each receiver misses the stream, then check show ip rpf 2.2.2.2 on R3.
The RPF neighbour of R3 is not a PIM neighbour: enable ip pim sparse-mode on R2 GigabitEthernet0/1.
R4 hears no IGMP report (show ip igmp groups is empty): enable ip pim sparse-mode on R4 GigabitEthernet0/1, PC4’s LAN.
Ping 239.1.1.1 from SRC again: PC3 and PC4 both reply.
Before you start
Every multicast join follows the unicast route back toward the RP or source (RPF), and every link on that path needs PIM on both ends. A receiver’s IGMP report is only heard on an interface with PIM enabled, so a LAN without PIM looks like a LAN without receivers.
Concept
The RPF check: multicast follows unicast routes backwards
A router accepts a multicast packet only on the interface it would use to reach the packet’s source (reverse path forwarding), and sends joins out of that same interface toward the RP or source. The unicast routing table decides the path, so a missing route or a link without PIM breaks multicast even when ping works.
Concept
PIM sparse mode (ASM): shared tree, register, SPT
Sparse mode sends a stream only where it is requested. Receivers’ routers join a shared tree rooted at a rendezvous point (RP); the source’s router registers the stream to the RP; once the stream arrives, the receiver’s router joins the source directly on the shortest-path tree (SPT).
