Multicast Overview
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Alcatel-Lucent
Beta Beta
OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
MULTICAST OVERVIEW
Multicast is an efficient way to deliver traffic from one sender to many potential
receivers.
The varied multicast protocols in use today range from host protocols (IGMP,
MLD) to routing protocols (MOSPF, DVMRP, PIM-DM, and PIM-SM/SSM). Earlier
versions of multicast routing protocols suffered from limitations and scalability
issues of one type or another. For these and other reasons, the PIM-SM/SSM has
emerged as the most popular multicast routing protocol for most service providers
today. OmniAccess 5740 USG supports PIM and IGMP.
The OmniAccess 5740 USG software supports the following multicast forwarding
features:
• Packet forwarding based on (S, G) entry
• Packet forwarding based on (*, G) entry
• RPF checks for all multicast data traffic
• Acts as DR for directly connected sources
• Acts as DR for directly connected receivers
• Acts as a RP.
PROTOCOL INDEPENDENT MULTICAST (PIM)
PIM-SM is described in RFC 4601.
The PIM sparse mode is optimized for internetworks with many data streams but
relatively few LANs. It defines a Rendezvous Point (RP) that is then used as a
registration point to facilitate the proper routing of packets. When a sender wants
to transmit data, the first-hop router (with respect to the source) node sends data
to the RP. When a receiver wants to receive data, the last-hop router (with respect
to the receiver) registers with the RP. A data stream then can flow from the sender
to the RP and to the receiver. Routers in the path optimize the path and
automatically remove any unnecessary hops, even at the rendezvous point.
Protocol Overview
PIM-SM is a multicast routing protocol that can use the underlying unicast routing
information base or a separate multicast-capable routing information base. It
builds unidirectional shared trees rooted at a RP per group, and optionally creates
shortest-path trees per source.
PIM-SM protocol is for efficiently routing multicast groups that may span wide-
area (and inter-domain) internets. This protocol is called Protocol Independent
Multicast - Sparse Mode (PIM-SM) because, although it may use the underlying
unicast routing to provide reverse-path information for multicast tree building, it is
not dependent on any particular unicast routing protocol.
PIM relies on an underlying topology-gathering protocol to populate a routing
table with routes. PIM protocol uses multicast/unicast routing table to get RPF
neighbor. PIM sends Join/Prune messages to RPF neighbor. Data flows along the
reverse path of the Join messages. Thus, in contrast to the unicast RIB, which
specifies the next hop that a data packet would take to get to some subnet, the
MRIB gives reverse-path information and indicates the path that a multicast data
packet would take from its origin subnet to the router that has the MRIB.
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