Objectives
The simulator ticks each one as soon as the network meets it.
Enable ip multicast-routing on R1, R2, R3 and R4.
Enable ip pim dense-mode on every addressed interface of the four routers.
On PC3 join group 239.1.1.1 (join 239.1.1.1).
From SRC ping 239.1.1.1: PC3 replies.
On R2, show ip mroute: the (10.1.1.10, 239.1.1.1) entry forwards to R3 and prunes toward R4.
Before you start
PIM-DM assumes every network wants the stream: the first packets flood out of every PIM interface that passes the RPF check, and routers with no receivers send a Prune back. IGMP runs automatically on PIM interfaces, so the receiver LAN must have PIM enabled too. The RPF check accepts a packet only on the interface the unicast route to the source uses.
Concept
IGMP: how a receiver asks for a group
A multicast group (224.0.0.0/4) is an address many receivers can listen to at once. A host tells its local router it wants a group with an IGMP membership report; the router then pulls the stream toward that LAN. IGMPv2 can only name the group; IGMPv3 can also name the source.
Concept
PIM dense mode: flood and prune
Dense mode assumes every network wants the stream. The first packets flood out of every PIM interface that passes the RPF check, and routers with no receivers downstream send a Prune back, so the flood shrinks to the branches that need it. The flood repeats every three minutes.
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.
