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Technical information about how AARNet runs the multicast protocol

Multicast allows a single source to simultaneously exchange traffic with many other machines.

Multicast is commonly used for broadcasting - a single computer can provide a audio or video feed to thousands of listeners. Multicast can also be used for group videoconferencing, such as between the Access Grid immersive videoconferencing rooms.

Access by sites to multicast traffic requires a request to the AARNet Network Operations Centre.

AARNet's multicast service uses the IETF's inter-domain multicast protocols, namely: MBGP, PIM-SM and MSDP.


To allow multicast traffic to take a differing path from unicast traffic, BGP runs a distinct multicast address family. BGP which seeks to negotiate this address family is named “multiprotocol BGP” or MBGP. If a site does not wish to engineer multicast and unicast traffic doesn differing paths then simply use the same routing policies for unicast and multicast BGP route filtering.


Protocol Independent Multicast (PIM) uses the unicast forwarding table (populated by some unicast routing protocol) to build the reverse path forwarding tree for each source. Multicast traffic is forwarded down into this tree.

Protocol Independent Multicast – Sparse Mode (PIM-SM) is a refinement where sources explicitly register with the PIM-SM rendevous point. This allows the tree to be built correctly from the beginning, thus avoiding forwarding multicast traffic down paths where there are no participants. AARNet runs PIM-SM internally. We recommend that sites do the same.

MSDP and Rendevous Points

  • To join two PIM-SM networks their rendevous points need to exchange data about active sources. 
  • The Multicast Source Discovery Protocol (MSDP) does this.
  • The AARNet network runs two rendevous points, at and

Administrative and TTL scoping

AARNet implements administrative scoping.

  • Traffic sent to multicast administratively-scoped addresses is discarded as detailed in RFC2365. Additional scoping is done.
  • Traffic sent to is contained within AARNet customers.
  • Traffic sent to is contained within AARNet customers within a state.
  • Traffic sent to is contained within a site.
  • AARNet does not implement TTL scoping.

Source-specific multicast

  • AARNet implements source-specific multicast, for both IPv4 SSM addresses and IPv6 addresses.


  • AARNet implements multicast for IPv6.

Quality of service and multicast

  • Multicast is only supported in the Best Effort and Scavenger classes. Multicast traffic which uses another class may be discarded.

Network abuse traffic

  • Traffic with a multicast source address or with an undefined multicast destination address is dropped.


  • Multicast traffic incurs identical charges to unicast traffic.