Multicast (IGMP / PIM)

Efficient one-to-many delivery — sent once, replicated by the network only where needed. Class D is 224.0.0.0/4.

// group ranges

224.0.0.0/24	Local network control (link-local, not routed)
224.0.1.0/24	Internetwork control
232.0.0.0/8	Source-Specific Multicast (SSM)
233.0.0.0/8	GLOP (RFC 3180)
239.0.0.0/8	Administratively scoped (private)

// well-known groups

// Layer 2 (MAC) mapping

239.142.57.6 → 01-00-5E-0E-39-06

11101111 10001110 00111001 00000110 → 0000 0001 · 0000 0000 · 0101 1110 · 00001110 00111001 00000110

// IGMP versions

IGMPv1	Original spec — join only, no explicit leave
IGMPv2	Adds leave messages + querier election
IGMPv3	Adds source filtering (used by SSM)

// PIM modes

Dense Mode	Flood then prune — tree starts everywhere
Sparse Mode	Built from an RP out to source & receivers
Sparse-Dense	Per-group: sparse if an RP is known, else dense

// distribution trees

Source-Rooted (SPT)	Shortest path from the source to each receiver
Shared Tree	Common links rooted at a Rendezvous Point (RP)

// terminology

IGMP	Hosts join groups by signaling local routers
IGMP Snooping	Switch inspects IGMP → forwards only to members
PIM	Protocol Independent Multicast — routes the traffic
RP	Rendezvous Point — shared-tree root (sparse mode)
RPF	Reverse Path Forwarding — loop check vs unicast route
CGMP	Legacy Cisco switch protocol (deprecated)

pim / igmp · cisco ios

ip multicast-routing
!
interface Gi0/0
 ip pim sparse-mode
 ip igmp version 3
!
! Static rendezvous point
ip pim rp-address 10.0.0.1

verify

show ip mroute
show ip pim neighbor
show ip pim rp mapping
show ip igmp groups