Layer 2 Multicast

• 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

Configuration:

• ‘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 239.0.0.0
      • range 237.0.0.0 238.0.0.0
      • 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.