Multicast and Multicast Routing

Unicast vs Multicast

  • Routed via PIM ( Protocol Independent Multicast ).
  • Always UDP-Based ( Video, Audiostreams, some kinds of data ).
  • Typical network challenges ( QoS, Security, Bandwidth consumption).
  • Began as a ‘speciality’ technology, becoming heavily adopted in modern times.

Multicast Addresses

  • Multicast group members receive all data center to a group.
  • Multicast IPv4 Range: [ ]
    • Filtering tip:
      • Last 23 bits of IP mapped to MAC
        • Can lead to overlapping addresses

Multicast Routing

  • Routing based on source address.
  • Multicast sources can just ‘start’ transmitting.
  • Routing is called reverse path forwarding ( RPF ).
  • Clients jump into a stream with a JOIN Message.
  • (S,G) Pair
    • Source, Group
  • Shortest Path Tree (SPT), eats resources, shortest path
  • Shared Tree – Uses rendezvous point ( RP) less resources, sub-optimal paths
  • Switches flood multicast by default
  • Add IGMP Snooping

Multicast Routing

  • Only modes use to be PIM SparseMode and PIM DenseMode
  • PIM-Sparsemode
    • Uses a rendezvous point ( usually a router )
      • Rendezvous points uses a shared tree.
    • Isn’t flooded but requested
    • PIM-SM is now PIM Any-source Multicast. Uses a Shared Tree , Source tree combo
  • PIM-DenseMode
    • Floods the network
    • Prune messages from sources to cut the stream
    • A lot of multicast sources can impact the network

Mirrored paths with Bidirectional PIM

  • Used for things like Teleconferences
  • Rendezvous Point ( RP ) is selected and everything is routing through it
  • ( * , G ) is used for all traffic
  • Only shared tree is used

Direct Contact with Source Specific

  • Multicast receiver gets server address from outside source
  • Outside source delivers (S,G) Pair
  • Requires requesters to use IGMP V3 ( asks for S,G instead of just G )

Finding your Rendezvous Point

  • Anycast
    • IPv6 method works for IPv4 ( DNS / Multicast ).
    • Assign the same address and let the routing table figure it out who is close.
  • Static RP
  • Auto RP
    • Cisco Proprietary
    • Holds election for C-RP ( Canidate Rendezvous Points)
      • uses for the election
      • Highest IP address wins
      • the router will announce it’s the canidate to
  • Bootstrap Router
    • Industry standard method implementing Auto RP
    • Holds an election
      • Canidate Boostrap Router
      • Priority is configured on C-RP. The C-BSTR sends the list of C-RP
      • The other router figure out which C-RP to use based on a hashing algorithm.

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