• Ethernet Multicasting
    • Supports L2 multicast natively.
      • Multicast bit in 48-bit address: lowest bit in the first byte.
      • Anything that starts with 01-
    • Multicast addresses used for various purposes.
      • CDP, L2 protocol tunneling.
      • Allocated by IEEE.
    • IPv4 addresses map to MAC to forward on LAN
      • Allows L2 switches to forward multicast intelligently.
    • MAC address range
      • 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF
        • First 25 bits fixed
        • Last 23 bits mapped from IPv4 address.
    • Implies overlap of addresses.
  • Switches treat unknown unicast and multicast like broadcast
    • Multicast traffic flooded out all ports in broadcast domain.
    • IGMP Snooping needs to be used to avoid this.
  • IGMP Snooping:
    • Switch listens for IGMP Report/Leaves
      • L2 device inspects L3 frames.
      • Extracts group address reported.
      • Prunes unneeded Multicast
    • Multicast Routers have to be discovered
      • Routers need to process all multicast traffic.
      • Switch listens to PIM messages.
        • PIM Snooping


  • ‘ip igmp snooping’
  • ‘ip igmp snooping vlan x’
  • Statically assign port to group:
    • ‘ip igmp snooping vlan <x> static <ip> interface <interface>

  • IGMP Snooping and STP
    • STP TCN may signal receiver moving.
      • After TCN event switch floods all multicast groups out all ports.
      • ‘ip igmp snooping tcn flood query count <count>’
      • Above command means flood until <count> query intervals have expired.
    • Disabling flooding during TCN
      • ‘no ip igmp snooping tcn flood’
  • IGMP Profiles:
    • access-group only on L3 interfaces.
    • Profile allows IGMP access-control at Layer 2.
      • Profiles are either in permit or deny mode.
      • Permit mode allows specified groups and blocks all others.
      • Deny mode blocks specified groups and allows all others.
      • Configuration:
        • ip igmp profile 1
        • permit
        • range
        • range
        • int gig1/0/45
          • ip igmp profile 1
  • IGMP Throttling
    • Limits amount of groups joined on interface
      • ip igmp max-groups NN
      • ip igmp max-groups action <deny|replace>
        • New groups are either denied or replace old ones.

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