Adjacent Node
Networking, explained. No BS.

IPv4 Multicast

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What It Is

IPv4 multicast sends one stream from a source to many receivers without copying the packet for every receiver at the source. Hosts join groups with IGMP, switches constrain flooding with IGMP snooping, and routers build multicast forwarding state with PIM.

Multicast is still used for market data, video distribution, paging, industrial systems, discovery protocols, and some service provider designs. It is also easy to break because it depends on Layer 2, Layer 3, unicast routing, group membership, and source reachability all being correct.

Address Ranges

Range Purpose Notes
224.0.0.0/24 Local network control Not forwarded by routers
224.0.1.0/24 Internetwork control Globally scoped control groups
232.0.0.0/8 Source-specific multicast SSM, uses source plus group
233.0.0.0/8 GLOP addressing Legacy AS-based allocation
239.0.0.0/8 Administratively scoped Private multicast space

Watch out: 224.0.0.0/24 is link-local control traffic. Do not use it for application streams.

Layer 2 Mapping

IPv4 multicast maps into Ethernet MAC addresses beginning with 01:00:5e. Only 23 bits of the multicast IP address are mapped, so multiple IPv4 multicast groups can map to the same Ethernet multicast MAC.

Design note: IGMP snooping matters because Ethernet multicast is otherwise flooded like broadcast on many switches.

Core Protocols

Protocol Role
IGMPv2 Host group joins and leaves for any-source multicast
IGMPv3 Adds source filtering for SSM
IGMP snooping Switch learns which ports need multicast
PIM sparse mode Builds multicast trees only where receivers exist
PIM dense mode Floods then prunes, mostly legacy
PIM SSM Uses source-specific trees, no RP needed

Modern note: Prefer PIM sparse mode or SSM. Dense mode and sparse-dense mode are mostly legacy troubleshooting topics now.

Trees And State

State Meaning Common Form
(*,G) Shared tree for any source to group G RP-based sparse mode
(S,G) Source tree from source S to group G SPT or SSM
RPF Reverse path forwarding check Source must be reachable via expected interface
RP Rendezvous point Shared-tree meeting point
SPT switchover Receiver path moves from RP tree to source tree Normal in sparse mode

Watch out: Multicast follows the reverse of the unicast route to the source or RP. Bad unicast routing creates bad multicast behavior.

Design Choices

Choice Recommendation
Application model Use SSM when receivers know the source
Group range Use 239.0.0.0/8 for private enterprise groups
RP design Use Anycast RP or a resilient RP design for ASM
Switch behavior Enable and verify IGMP snooping and querier placement
Boundaries Filter multicast at site and security boundaries
TTL Use TTL deliberately for scope control
Wi-Fi Avoid high-rate multicast unless WLAN design supports it

Cisco IOS/IOS-XE Examples

PIM sparse mode with a static RP:

ip multicast-routing
!
interface Loopback0
 ip address 10.255.0.10 255.255.255.255
 ip pim sparse-mode
!
interface GigabitEthernet1/0/1
 description Routed multicast link
 ip address 10.0.12.1 255.255.255.252
 ip pim sparse-mode
!
ip pim rp-address 10.255.0.10 239.0.0.0 255.0.0.0

IGMP access VLAN:

interface Vlan50
 description IPTV receivers
 ip address 10.50.0.1 255.255.255.0
 ip pim sparse-mode
 ip igmp version 3

Switch snooping:

ip igmp snooping
ip igmp snooping vlan 50

SSM range:

ip pim ssm range 232.0.0.0 255.0.0.0

Notes:

  • Static RP is simple, but not automatically resilient.
  • BSR, Auto-RP, and Anycast RP syntax varies by platform and design.
  • Use ACLs for group boundaries and source controls where appropriate.

Troubleshooting

Symptom Check Likely Cause
Receiver sees nothing IGMP group, VLAN, snooping, PIM state Receiver join not reaching router
Source traffic not forwarded RPF interface and unicast route to source RPF failure
Works on same VLAN only PIM neighbor and RP/SSM state Routed multicast not built
Flooding on switch IGMP snooping, querier, unknown multicast Switch lacks group membership
SSM fails IGMPv3, source address, SSM range Receiver did not join (S,G)
ASM fails RP mapping and reachability No valid RP
Intermittent stream Drops, TTL, duplicate RPs, Wi-Fi airtime Path or scope instability

Commands

show ip igmp groups
show ip igmp interface
show ip igmp snooping groups
show ip pim neighbor
show ip pim interface
show ip pim rp mapping
show ip mroute
show ip rpf <source-ip>
ping <group-ip>

Expected clues:

  • Receiver VLAN shows IGMP membership for the group.
  • PIM neighbors exist on routed multicast links.
  • show ip mroute has the expected (*,G) or (S,G) state.
  • RPF points toward the source or RP as expected.
  • Switch snooping table includes the receiver ports.

Watch Out

  • Do not skip unicast routing checks. RPF depends on them.
  • Do not use multicast link-local control ranges for applications.
  • Do not assume Wi-Fi handles multicast like wired Ethernet.
  • Do not leave private multicast groups unbounded across WAN or campus edges.
  • Do not troubleshoot only on the receiver side. Check source, RP, and RPF.

References