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Multicast Route construction in the Core Base Tree (CBT) method

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• First a note:
  • Throughout the discussion, I will assume that all routers in the network are multicast enabled.

  • In reality, there are still plenty existing routers that are not multicast enabled.

  • However, that does not affect the discussion on multicast routing.

    You have to consider the multicast routing only on the multicast enable logical network.

  • For example, in the following network with 10 router of which only 4 are multicast enabled:

    

    the multicast enable logical network is as follows:

    

• Core Base Tree multicast routing method:
  • When a multicast enable router receives a IGMP join message for a new multicast group, it will process the join message by creating/adding the interface to the multicast route entry and possibbly forward a craft message towards the core node.

  • A router that receives a craft message for a multicast group G will create/add the sending router's ID to the multicast table.

  • If the router is not yet part of the core base multicast tree, the router must
    • forward the craft message towards the core node and
    • add the next hop router to the multicast route entry.

  • A router that is already part of the core base multicast tree will not forward craft messages further.

• Example:

  Initial state...

  

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  When a receiver at C's subnet joins:

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  See how multicast packets are routed:

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• Continue with example... when a host served by router D joins:

  

• And when a host served by router F joins:

  

  • Note that since router F is part of the core base tree, after processing the join/craft message, it does not need to forward the join/craft message toward the core

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• DIfference Multicast and Unicast Routing
  • In unicast, there is always a route in the unicast routing table for any (valid) destination

  • In multicast, a multicast route to a receiver is created only when that receivers joins a multicast group.

    I.e., the multicast routing tables in routes are being manipulated by multicast receivers !!!

• How to become part of the Multicast network ?
  • Host computers are typically connected to a (Ethernet, FDDI) LAN

  • In order to provide multicast on a (broadcast) LAN, a multicast enabled router must be present on that LAN.

  • Such a multicast enabled router can process IGMP (Internet Group Management Protocol) messages

  • A host on a broadcast LAN (Ethernet, Token ring) that wants to join a multicast group will announce their multicast group memberships to the multicast router using a special IGMP-JOIN protocol message:

    

    The multicast enable router receives the IGMP join message and forward special route update messages to other nodes to extend the multicast forwarding tree to accommodate new receivers on its LAN.