About Multicast Routing
Multicast routing is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to thousands of corporate recipients and homes. Applications that take advantage of multicast routing include videoconferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.
Multicast routing protocols deliver source traffic to multiple receivers without adding any additional burden on the source or the receivers while using the least network bandwidth of any competing technology. Multicast packets are replicated in the network by ASA enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, which results in the most efficient delivery of data to multiple receivers possible.
The ASA supports both stub multicast routing and PIM multicast routing. However, you cannot configure both concurrently on a single ASA.
The UDP and non-UDP transports are both supported for multicast routing. However, the non-UDP transport has no FastPath optimization.
Stub Multicast Routing
Stub multicast routing provides dynamic host registration and facilitates multicast routing. When configured for stub multicast routing, the ASA acts as an IGMP proxy agent. Instead of fully participating in multicast routing, the ASA forwards IGMP messages to an upstream multicast router, which sets up delivery of the multicast data. When configured for stub multicast routing, the ASA cannot be configured for PIM sparse or bidirectional mode. You must enable PIM on the interfaces participating in IGMP stub multicast routing.
The ASA supports both PIM-SM and bidirectional PIM. PIM-SM is a multicast routing protocol that uses the underlying unicast routing information base or a separate multicast-capable routing information base. It builds unidirectional shared trees rooted at a single Rendezvous Point (RP) per multicast group and optionally creates shortest-path trees per multicast source.
PIM Multicast Routing
Bidirectional PIM is a variant of PIM-SM that builds bidirectional shared trees connecting multicast sources and receivers. Bidirectional trees are built using a Designated Forwarder (DF) election process operating on each link of the multicast topology. With the assistance of the DF, multicast data is forwarded from sources to the Rendezvous Point (RP), and therefore along the shared tree to receivers, without requiring source-specific state. The DF election takes place during RP discovery and provides a default route to the RP.
If the ASA is the PIM RP, use the untranslated outside address of the ASA as the RP address.
Multicast Group Concept
Multicast is based on the concept of a group. An arbitrary group of receivers expresses an interest in receiving a particular data stream. This group does not have any physical or geographical boundaries—the hosts can be located anywhere on the Internet. Hosts that are interested in receiving data flowing to a particular group must join the group using IGMP. Hosts must be a member of the group to receive the data stream.
Multicast addresses specify an arbitrary group of IP hosts that have joined the group and want to receive traffic sent to this group.
Multicast routing supports clustering. In Spanned EtherChannel clustering, the primary unit sends all multicast routing packets and data packets until fast-path forwarding is established. After fast-path forwarding is established, subordinate units may forward multicast data packets. All data flows are full flows. Stub forwarding flows are also supported. Because only one unit receives multicast packets in Spanned EtherChannel clustering, redirection to the primary unit is common. In Individual Interface clustering, units do not act independently. All data and routing packets are processed and forwarded by the primary unit. Subordinate units drop all packets that have been sent.
For more information about clustering, see ASA Cluster.