Multicast Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.



  • – is multicast range
    • – reserved for network protocols / local network control block
      • 224.0.013 is PIMv2 and is IGMPv3
  • – globally scoped addresses for internet etc
    • Network Time Protocol (NTP)
  • – is private addresses / limited scope addresses / administratively scoped block
  • PIM – Protocol Independent Multicast. Creates a multicast tree. Source sits on top of the tree & sends Mcast stream. Recipients on logical branches. If no downstream router have mcast group , then that router does not forward mcast traffic
  • PIM-DENSE – No RP. Uses prune and flood technique to build tree. Initially floods network, if routers do not need it. It sends Prune to upstream. Ideal if mcast source & recipient are physically close. Few senders, all routers can forward mcast traffic.
  • PIM-SPARSE – uses RP. Ideal for mcast routers that are not close. Multiple and simultaneous streams. Stream not constant. Adds interface based on join message
    • RP can be statically configured on each router or can elect and find dynamically.
    • PIMV1 – Static or Auto-RP
    • PIMv2 – Static, Auto-RP or bootstrapping
    • Sparse-Dense Mode: If RP = Sparse. If no RP = Dense. Must define RP otherwise it will default to Dense mode
    • IGMP – Internet Group Management Protocol – Allows a host to join a multicast group
      • Ver 1 – host sends a membership report to it local router. Shows what mcast group the host wants to join. Router then queries, sends general query every 60 seconds to ask other hosts if they want to join that group to. Inefficient, too much traffic / renewing membership every 60 seconds. If host leaves mgroup, router waits 3 minutes until then keeps sending mcast traffic – hence inefficient
      • Ver 2 –Has a leave group message, when host quits. However router sends general query to all host if they want to continue the  group. Host replies with report. Lowest ip address router is the querier. Still sends general queries = inefficient
      • Ver 3 – Source filtering
      • Querier – Router with the lowest IP address
      • DR – Router with the highest IP address. NO DR on a POINT-TO-POINT link
      • RPF – Reverse Path Forwarding unicast routes towards destination. Multicast routes away from it source. Destination is a multicast group. Router needs to know how to route back to the source and what are the downstream paths.
      • RPF Check – inspects incoming multicast packet. If it arrived on the upstream interface, the packet is forwarded otherwise it is dropped#
      • IGMP Snooping – helps layer 2 switches. Listen to host reports, records multicast Mac addresses and port to determine which ports require stream rather than flood all ports. CPU Intensive, only ideal on higher end switches
      • CGMP like IGMP snooping helps layer 2 switches. It not as CPU intensive. Need to enable on both the router and switch
      • MAC Address – multicast MAC starts with 01-00-5e. The rest is the IP address (last 3 octet) of the multicast. E.g.
        • = 11100000  00000000 00000001 00001100
        • Hex                                              00               01             0C
        • = 01-00-5e-00-01-0c
        • = 01-00-5e-00-01-0c

Convert 01-00-5e-04-43-AC in IP

  • X. 4 (04). 67 (43). 172 (AC)
  • 1110xxxx. 00000100             01000011             10101100
  • X                             4                              67                           172
  • 1110xxxx. 10000100             67                           172
  • X                             132                         67                           172


Enables router for multicasting

  • R1 (config) # IP Multicasting-routing

RP Address

  • R1 (config) # IP pim rp-address

Enable PIM on interface

  • R1 (config-if) # IP pim sparse

Shows multicast neighbours . IP add, Interface, Version etc

  • R1# Show ip pim neighbour