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: [ 224.0.0.0  239.255.255.255 ]
  • 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 224.0.1.39 for the election
    • Highest IP address wins
    • the router will announce it’s the canidate to 224.0.1.40
• 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.