CCIE RS – Written – L2 Multicast – Implement and Troubleshoot IGMP

Implement and Troubleshoot IGMP

Specifies how a host can register with a router in order to receive specific multicast traffic

  •  To inform a local multicast router that a host wants to receive multicast traffic for a specific group
  •  To inform local multicast routers that a host wants to leave a multicast group (in other words, the host is no longer interested in receiving the multicast group traffic)




RFC 1112

  • Membership reports are issued by hosts that want to receive a specific group (GDA)
  • Membership queries are issued by routers at regular intervals to check whether there is a host interested in the GDA
  • Host membership reports are issued either unsolicited or in response to a membership query
  • Membership queries are sent by routers to all multicast address
      • All multicast capable hosts
    • Host for each group must respond to the query or the router will stop forwarding all traffic for that GDA after 3 attempts
  • IGMPv1 has no leave mechanism

Default Query Interval – 60 Seconds




RFC 2236

  • Host should send a leave message to destination
    • All multicast capable routers




RFC 3376

  • Backwards compatible with v1 and v2
    • V1 membership report
    • V2 membership report and leave group
  • Source Specific Multicast (SSM)



IGMP Snooping

  • Allows switch to listen to IGMP conversations between host and router
  • When switch hears an report for host to multicast group it adds a host port number to the GDA list
  • When it hears a IGMP leave it removed the hosts port from the CAM table entry
  • Learning Router Port
    • IGMP Membership query send to 01-00-5e-00-00-01
    • PIMv1 hello send to 01-00-5e-00-00-02
    • PIMv2 hello send to 01-00-5e-00-00-0d
    • DVMRP probes send to 01-00-5e-00-04
    • MOSPF message send to 01-00-5e-00-05 or 06

Enabled on per-VLAN basis



IGMP Querier

Sends periodic IGMP queries that trigger IGMP report messages from hosts that want to receive IP multicast traffic



IGMP Proxy

Enables hosts in undirectional link routing (UDLP) environment tha are not directly connected to a downstream router to join a multicast group sources from an upstream network

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